Only needs to be attacked, no damage required. 

Avenging doesn't help much. The CB has only a crit chance of 15%, so there won't be many counter attacks from that set.

Retaliation > Avenging.  Pretty niche set, but is good on a champ like Vrask.  

Think of Avenging as the same set, but for Arena purposes only. In PVE, retaliation is better because enemies dont proc crits that often. Personnally, i sell all the avenging. I keep 2 sets of retaliation for 2 champs i use sometimes in arena, sometimes in PVE.

Something else to note - retaliation gets a damage penalty. Avenging is the clear favorite for arena, where you're far more likely to be 

Thanks all! 🙂