Each time you assist another player or kill a boss yourself you boss kill count go up. When it reach 100 the 8.7mill bosses start to show up.
The higher the kill count the higher chance of the 8.7 mill boss.

If a player didn't assist other players at all he would reach 1.2 - 1.5 mill EP before getting his first 8.7 mill boss.

Assisting give good rewards but it gives you the 8.7 mill boss too.