Currently, as Summoner is, it is a complete downgrade from the Non-subclasses Medic/Monster Master. The fact that my level 129 Summoner gets lower Scores, less XP gain, less usefulness to the team in terms of healing, less survivability AND less damage output compared to my level 85 Medic/Monster Master speaks for itself.

As is, the Upsides to the Summoner is:
14% more pet DB (which is barely anything)
10 more Monsterpoints (which I believe is still 5 lower than old DC's "Extreme Monsters" Subclass that nobody complained about)
Bigger variety of pets (altough many overpriced both in MP and Adren, and some which barely does anything)
50% more pet HB (although this makes no difference since the damaging pets has such little HP already they die with this maxed easily)

The Downsides however:
40 HB less overheal
5 levels less of 'Experienced Healing' (So only level 2 which gives little to no XP)
A whopping 50% reduced DB (Incredibly high tradeoff in damage for pets who do not balance this by their damage output)
3 levels less of Advanced Damage Reduction (meaning I and my pets take 12% more damage than a Non-Subclass Medic/Monster Master)
The most expensive subclass in the game. Funnily enough also seems to be the subclass that gets the least bang for the buck per point spent.

I have tested pretty much every combination of pets, on a majority of the maps, both with high levels players, low level players and no players on the server and the result is always the same: The Summoner is cool, but not useful.
I think the main reasoning for this is the fact that the Ice/Fire update made the server much harder, and the Summoner has not been rebalanced with this in mind.

These are just some ideas, I do not mean that necessarily all of them should be done, just some food for thought:
My suggested changes for balancing the Summoner:

25 MonsterPoints (which means +5 MP from current Summoner)
- Will open up possibilities of more Pet variations (and obviously slightly more pets overall)

Alter "Monster Damage Bonus" ability to "Advanced Monster Damage Bonus", where each level will increase Pet Damage by 4%, instead of the current 2% per level (Don't just increase the levels of "Monster Damage Bonus", this class is already the most expensive subclass in the game)

Introduce 'Pet Damage Reduction'. Give pets 1-2% Damage Reduction per level. Also, this should be integrated with the "Advanced Monster Damage Bonus" ability, to reduce overall costs.

Unbreakable Summoning Charm. Much Like AM has unbreakable MWM, why not introduce an unbreakable Summoning Charm for Summoner?

Remote Pet Boosters; Basically like the AMs Remote Boosters, but they only work for Pets. That way you can grant your Pets extra damage on waves where they can likely kill a lot and safety on waves where they are likely to die. This will also give the Summoner something to use the Adrenaline on once the pets are summoned. Since they only work on pets, 50 Adren (Like Craftsman for Players which are far deadlier) seems fair. Perhaps even slightly lower. 10-15 XP per boost (which still is lower than Craftsman). Thus the Summoner gets some XP gain back both by boosting their pets, but a bit of extra XP gain by the effects of the boost (in terms of damage/safety)

Introduce some new pets;
Slug - Unique pet, unique attack. Allow 3 since they are quite slow and clumsly, and the fact they will miss their targets on big maps.
SlimeKrall (Green Krall that shoots Link Lasers) - Fun pet with good Damage but low mobility and Health. Allow 4
Invisible Krall that shoots red (Same as Green Krall)
Cosmic Krall - Very good Damage, very low HP, good mobility. Allow 4
Invisible Brute/Behemoth (Good damage, Low mobility. Good as bodyguards) Allow 3
Cosmic Skaarj (Very good Damage, very low HP, good mobility, but more aggressive than Krall) Allow 3

Introduce "SuperPets" - Basically High MP/Adren cost for Big and powerful pets (perhaps powerful who knows until they are tested as pets)
Queen. The one for 12MP and 150 Adren cost which is currently on Lvl 19 of Loaded monsters (which currently is disabled, so it gives you nothing)
Ice/Fire Lord - The Deemer shooting warlords. I'm not sure if they freeze / burn players or if their Deemers can be shot down by allies. Should probably only allow 1, at slightly more than 50% of max MP. Fire slightly more than Ice.
Tech Titan. Again, not sure if Mines will target Allies or actual monsters. Again, allow 1 only, and a medium pet. Unique pet especially with the mines. Also should serve as a good bodyguard for the Summoner.
Cosmic Titan. Should cost close to all, if not all MP available probably. Is mobile, so would function well on big maps as Summoner does not do very well outdoors currently (even worse outdoors than indoors)
Arctic Skaarj. Should cost all MP. It has very high damage, but only 1 target at a time, making sprees hard to get. Also has low Health compared to others, and it's only 1 pet making you very reliant on keeping that particular pet alive.

These changes however, I feel is imperative to look at, since some pets are currently overpriced:
FlakBag (8MP, 90 Adren): This pet gets stuck alot, misses targets alot so is basically only useful against small monsters. Against big targets they really don't do that much damage. This pet is not very useful, but is nice to have the option to summon it.
- 4/5MP, 45-50 Adren

Gold Titan (11MP, 135 Adren): The only pet that doesn't die immediately on Waves 13+. Literally the only one and it STILL dies fairly quick. Should Allow 2 of these as bodyguards to prevent the Summoner from dying immediately since monsters like to aggro them. Their damage output is low since they often stand around and do nothing, incredibly rarely throw rocks, but is useful when they do (which btw, Titans and Icetitans are currently immune to their rocks), and a melee attack which isnt overly powerful
- 10MP, 135 Adren

Cosmic Warlord (10MP, 150 Adren): Good pet, but is a bit limited to what it can be used for. Laughs after every kills (making them not shoot for 3-4 seconds after EACH kill). Will also laugh midair, meaning they will always fall to their death in maps with deathfalls. Also, smaller explosion than Cosmic Brutes. And since it's Cosmic, it does not have a lot of HP, and 150 Adren which is very much for a Summoner is a steep price for a pet that can be killed by 1 Geodude instantly.
- 10MP, 100 Adren

Lava Skaarj (14MP, 150 Adren): High damage, Smart AI, but unfortunately likes to follow you instead of shooting if it can see you. If you try to stay out of its eyesight, it does not survive long because it needs healing to survive, which severely limits its use. Also has an arched shot meaning it will miss in outdoor maps on a vast majority of monsters, making it shoot something it can't reach. A good pet, but there are much better options at 14MP, rendering it somewhat obsolete.
- 11-12MP, 130 Adren)

Skaarj Sniper (7MP, 90 Adren): One of the very few pets that works outdoors. (And pretty much the only pet that works on huge maps). Thus, 2 and something worth 6Mp (Totalling 20MP), is still quite low damage output. Also consider that it shoots slowly and does actually miss, so DPS isn't exactly staggering.
- 6MP, 75 Adren

When taking all of this in mind, try to think about it from a Summoner's perspective: You rely on your pets to kill for you, but this also means the pets has to kill the monsters around you or you're dead. 14% extra DB is not horrible, but when your pets are all attacking monsters across the map (or doing absolutely nothing) while you're being attacked, it sure would be nice to have something that barely misses, has high damage output, is free and has 1000 HP (Like the Engineers' sentinels). Even more damage when Subclassed, heck the Engineer weaponry's only downside is 3 blocks less (While big increase in damage both in turrets and sentinels). Lower weapon damage sure, but who cares, you have a turret than 1 shots a Firetitan when linked. AND it's free to summon. And you're behind Defense sentinels that grant you 100% DR most of the time (also free). And you're behind blocks that the monsters cannot easily get through.

My point is, a lot of other subclasses gets pretty much straight upgrades with no real downside (Extreme WM gets 20% Extra DB over normal WM, and all weapons magics maxed) for the expense of Adren cost being higher only on artifacts (meaning you can still Berserk) - Solution? Don't use artifacts you're fine without them)
Engineer Weaponry gets what 40% Increased Damage in turrets and a lot in sentinels too, better sentinels and better turrets, at the expense of what? 3 Blocks less and less weapon damage.
-Solution? Use Turrets like an Engineer is supposed to and find a basespot with 7 blocks. With creativity, I've found 4-5 Blocks can be enough, so you have an abundance of blocks.

As is, I'm struggling, REALLY struggling to get 3.5k XP per map. At level 129.
Whereas others, myself included, have reached 8-9k XP in 1 map, obviously on other classes.

Summoner's solution as currently is: Don't subclass into Summoner. Summoner cannot circumvent the 50% lower DB, 12% less DR, 40 HB less overheal and lower experience gain since Experienced Healing is only level 2.

- I've already increased the damage on pets from 2% per level to 5% for the 205B build, so that'll be up to 50% extra damage through pets. 50% less weapon damage will remain.

- I'll increase adren drip from 1 to 2.

- Although your pets get the abilities you have, including advanced damage reduction, I like the idea of having damage reduction for pets specifically, and not the player. That'll be a possibility.

- I've also noticed that the healing damage as summoner is weak -- maybe it's because I'm used to the Healer's healing output. I'll most likely increase the Summoner's healing output, especially because they have tanks like Gold Titans with large HP, but experienced healing will remain at 2.

- I've already considered adding in new pets to the mix including Slugs and Blast Kralls (the green ones). I'm already changing up the 'same species as' function so that they'll attack and/or damage their own kind more often. On top of that, I'm considering adding in a teleporting effect like the Nalis use so that 1) they get out of areas they might get stuck in, especially behind bases, and 2) they'll attack more often, and because most pets are projectile-based, they'll actually hit their targets more when teleporting near it.

- Other than those, I'm not entirely fond of increasing MP or the remote/charm artifacts. As you mentioned, summoners are the most expensive and we can instead lower the MP cost on pets -- if, that is. I can see three snipers, but I don't like the idea of two Titans. Large pets are self explanatory, they just block everything, which is good for you personally but a nuisance to everyone else. So if -- if I add Queens and Slugs as pets, you'll be able to summon one. No remote boosters- everything will be given to pets through abilities, no unbreakable charms.