Finn needs to survive as long as possible, so he can end up doing great dmg.
C-3PO will help greatly, since translation gives +30% max health, and enemies will constantly be inflicted with offense down, and additionally whenever he calls Finn to assist, Finn gains +20% stacking dmg, and with 3 stacks of translation he'll have +40% crit dmg. C-3PO exposes on basic as well, and can trigger exposes with his mass assist.
R2, with his number crunch will help by increasing max health and max protection, and will also increase Finn's offense. The stealth is also great for protecting weaker characters of course. And with C-3PO present, he'll also expose with his basic.
Holdo is very durable and provides taunt, healing and crit avoidance.
The best fifth character is probably resistance trooper, but some people might want to save him for their JTR team. Resistance pilot won't be quite as good, but is still very decent. Poe will be good with his taunt and aoe expose.
I guess my two resistance teams will be:
Finn, C-3PO, R2, Holdo, Resistance Trooper
JTR, BB8, Scavenger, Resistance Pilot, Rose Tico (or possibly Chopper or T3)