Okay I've tried defeating several bosses in the event and I can barely do any damage to them before all three of my party are defeated and it doesn't seem to matter if I play more offensively or defensively or how many health potion I use am I doing something wrong or not doing something I should?

Aaah, well I did a quick lookover your pets, and I have a few pointers to make.

1) All of your team should be at around the same level, otherwise one pet's gonna be falling behind and that one space will go to waste.

2) Your tank, Evergreen Guardian, has one point in Strength/Intelligence, when as a tank, his points should go to HP. With max-stats, it's a slightly different story, but you don't have those so...

2a) Also, be sure that your tank is the one in the middle on the screen. The mobs like to target that sylesti.

3) This is just an addon, but fighting will become so much easier once you hit level 30 and get that second attack.

I'm sure someone can go more in-depth for you, but these are what stood out to me.

Evergreen Guardian has two Int accessories- as an Earth tank he should have all Assassin gear. Tanks can technically work with Str/Int, but generally the first prof point is better put towards HP or Agi depending on style of play

Twilight Moon needs an Evoker relic (and... Evoker equips in general), and failing that an Illusionist. She has a Phys type relic now, meaning she's dealing base 87 damage when she attacks.
None of her equips are good for a Light Healer, though the Legendary Disciple accessory can have a pass as 'okay'

Mystic Twilight Moon needs a Myrmidon- as with Twilight Moon, she has the wrong type of equip, an Int relic on a phys element pet, and so is only dealing 45 base damage.
Replace all her equips ASAP, because uncommon equips are garbage anyhow.
Prof would probably work better as Strength, with the second prof point at lv30 put to either Str or Dex

Thanks guys for the advise once I get the items you guys have mentioned I'll be sure to add them to my team thank you again !!!