The reason why dark blob is so hard to beat is because of his multiple resistance. Therefore, even your hardest hits will do little damage. Just have the computer players attack dark blob while you keep a good distance and aviod being squashed. Eventually his shield will disappear, but only for a short time. It is in those periods that you attach with the stronger players.

ive never vsed dark blob only normal blob

Dark Blob is in the Astral Plain

you need to take out the statue and attack him after each one