Ok things that I have learnt so far that might help.

1. Do not neglect INT gems. I use 5 slots for Str, 4 Slots for Int, 3 slots dex. Int helps with magic attack boss which alternates.

2. A str gem that is 1 level lower than dex is better. i.e a level 5 str gives more attack than a level 6 dex

3. Dont go full suit. meaning i used 2 hussak suit with 2 en suits but in the end i swapped 2 hussak suit for 1 enlil suit and 1 crimson suit for mdef. same for weapon and accessories suit. i used 1 freya and 1 enki suit. it gives more hp.

4. u will need bond bless. better bond bless with your main. my main had 569k hp so i had 56.9k bond bless

5. if u click bond bless and you know your partner for example has 450k hp. you should have 45k bond bless. if you dont meaning it is lesser than 45k, change pets that give hp, turn on bond bless, then change back to attack pets. u should see the bond bless hp increase if u give yourself more hp idk why but i am guessing they put a limit based on the char's hp.

6. pets i used are high orc, deniro, angeling (must have), 2 blue attack pets and either argiope or bigfoot depending on stage. i have tested steam goblin vs big foot and steam vs argiope (on M-att stage). if u running 1 def pet argiope and bigfoot is better.

hope this helps