I need help with killing monsters

I try to kill a monster and i see tha massife damage on top of the monster, but it dosnt lose hp, heres a SS.

WHY WONT ANYONE REPLY I NEED HELP!

That happens to me to i think it is lag.

well i think it was m triple throw, i rebirthed and i dint have the same problem

It was prolly lag. Sometimes like in other private servers iunno if it happened in this one yet I like hit the monster and then it just doesnt die. Then I change channel or go in the portal and come back and it's gone so it probably died but the monster didn't disappear o.o;