You have to focus your gearing efforts on characters that will always be relevant and/or are useful in many areas. For example, gear Phoenix and you can use them for 2 legendary unlocks, for GW, and for TB. Resistance makes GW a breeze, is great for the tank raid, and can guarantee you 6/6 on a TB node.
As for arena, just gear the best toons. Most of the time the best teams are just an amalgam of the most OP toons. There was Threxus for a while and now it's just CLS, Zolo, R2, GK, and Thrawn. There is scarcely synergy, it's just throw in the strongest toons and you win.