@katasaur,
I would lose the Light Healer and swap it to Shadow, personally. I tried that in the begining, and came to the conclusion that the chance heals are insignificant at this bracket compaired to the damage you are taking. The shadow pet is going to put it's syphon effect onto the boss, and gain healing back from that anyway.

Looking at your pets, though. I think this may also be a gear problem. You seem to be gearing them all in a ton of Disciple. I'm sure this makes them more defensively stated, but they don't have as much firepower in their attack stat because they sacrifice having some of the numbers in their prefered stats while throwing them at the others. Ie: Your Magic DPS pet has a whole bunch of extra points going into Strength instead of more points to Intell. I would curb them back towards more specific statted gear, personally. ^^

Alright, I'll switch her to Shadow and go shopping for some new gear. Thanks so much!
Battling isn't really my strong point. xD;

Man, I've tried out the tips and such, but I'm beginning to think there's a problem with my team too :c
So far it has been working fine, but... I can't beat those two. Even though I get Katalyst down around turn 15, my Tank is around 20% Health at that point even after Mending, and I get Kimeira down to 32% max.

My team is the following:
Nocturnal, Water Tank
Ton, Earth Attacker
Enzian, Light Mage.

Like I said, so far I've been doing fine... But my golds going down the Heal-Drain.

@Lenexia,
My first suggestion is to improve your gear. The stat boosts may seem somewhat insignificant on the pet's page, but just improving one piece of gear can make a world of difference. I've got some spares I could throw your way if you'd like. :3

Second, try to find gear that focuses on Strength/Intelligence and Agility rather than Dexterity. Dex can be useful in very specific battles where quickness is essential (such as your next foe, Oogar), but I find concentrating on Agility and my pet's attack stat to be far more beneficial.

And don't forget to swap around your pets during battle to even out the damage! Your tank can only handle so much. ;3

Does anyone have advice for my battle group?

http://www.sylestia.com/view/pets/?petid=181180
Water fighter
5/5 in health
50 proficiency points in health
50 proficiency points in strength

http://www.sylestia.com/view/pets/?petid=182055
Water fighter
5/5 in health
50 proficiency points in health
50 proficiency points in strength

http://www.sylestia.com/view/pets/?petid=148569
Earth fighter
5/5 in health
100 proficiency points in health

definitely better gear your one guy has a blue item on him and purple is ok, but i suggest looking for some oranges if you can, also if u got some lvls 50's you can easily do a dungeon run for some gear.

Hey I figured it out:
obviously having a dark pets instead of light helps with this bracket, but I also found that u should definitely focus on Katalyst as suggested. Watch your pets though. If a swirly purple icon appears on ur pet, Kim is gonna use her special attack on them, you should block, otherwise not only does ur pet take alot of dmg, but Kim will heal what almost seems like 10% of her health, and trust me that got annoying. Don't worry tho, like with the petrifying affect, she won't do it until a few turns l8r. so you can final hit her afterwards, and there's still time for ur one pet to get his mana bk to 100% and final hit, b4 she attacks again, norm she uses her special on the middle, unless ur using an earth pet, then she might attack them if she has the target effect on her.

OMG, thanks SO much for that advice, I beat them finally!! ^^

so what does Krin mean when he says "buff and debuff"? *I don't speak or recognize that language ^.^

Buffs are any type of equipment, spells etc. that give your character a boost, debuffs mean to unequip or equip with something that has a lower rating than what is being used. Buff/debuff terms are used in a wide variety of games.
In the example for the Arena here, itis talking about equipment.