Don't bother using him as an attacker, because he can call an attacker to assist every other turn.
Pick from types of mods
health- make him tougher+ pass more prot
defense- make him tougher (without making him more vernable to %health effects like expose or chewie)
crit avoidance- make him tougher (but only against crits, which is a safe bet as they are the biggest hits in the game) this is also just an arrow primary, not a set.
speed- faster = more self healing + more assists called + more likely to get revive off
tenacity- harder to interrupt his assist calling and revives. He is tmr proof, but not stun/ability block proof
which of above depends on where you use him. For the best damage output, slap a speed set on him. For arena, you may want to consider tenacity or a crit avoidance arrow or similar. But those two things won't help in raids