Character Card System

I guess this would be the best option for most people then:

Deck 1: Jett lvl 120, Cannoneer lvl 120, Mechanic lvl 120 (6% ignore defense, 15% buff duration increase, 8% summon duration, 15 all stat 5% hp/mp (link skill), Jett's core (link skill), 4% exp lose reduction S rank +700 HP, +700 MP, +3 ATK, +3 M.ATK, +3% damage to bosses)
Deck 2: Mikhail lvl 120, Dark Knight level 120, Demon Slayer lvl 120 (4% weapon defense, Stance (link skill), level 30 koc buff, 4% hp, 3% Status resistance, boss dmg (link skill), 8% base hp into damage(range(?)) & S rank +700 HP, +700 MP, +3 ATK, +3 M.ATK, +3% damage to bosses)

And you should replace dark knight with kaiser (for the link skill, I obviously dont know what his card bonus is yet)

Total bonus: 1400 HP/MP, 6 M.Att/W.Att, 16% damage to bosses, 6% ignore defense, 15% buff duration, 8% summon duration, 15 all stat 9% HP 5% MP, Jett bonus, 4% lose reduction, 4% Weapon Defense bonus, Stance 50% of the time, 30 koc buff, 3% Status resistance & 8% base hp into damage. All this for only 6 char slots.

You would only need 7 slots if you want to get mikhail koc buff (since he obviously cant give his own koc buff to himself)