Each time you kill a boss or help kill a boss your kill count go up with 1.
The bosses you summon will have a level that are between half your kill count and you kill count.
With a kill count of 200 I got bosses with between 900k and 1900k hp at.
If I solo the bosses I get 1 kill per hour shareing them give me 2-3 kills per hour.

I got 1.7 Mill EP atm and I think I will end between in top 51 - 100. Japan servers have rewards upto 100 Mill EP you we only got rewards to 1 mill EP.
So EP don't matters much to me I rathere have some more rewards by shairing bosses.

For me 1 hit one rewards if better then 10 hits 2 rewards whith is what I get when I sole bosses