To answer your question, yes, round one is based on power/damage. Final round/to strike the final blow - this is based on # of times the heroic boss is approached and hit by a player. Final round has nothing to do with heroes. I think it's a number near 30 or 35. If z = number of players playing final round (regardless of power, heroes, etc.) and y = number of times each player approaches and hits the boss, then the equation would be z x y = ~33

I come (came, our server is being merged) from a small server, many times 2-4 people banishing, sometimes solo even.