level 3 unbeatable

I have experienced a bug where I can not complete the third level the objective to defeat marcus will not complete I destroyed his core and my minions took it over for me but the objective stays i have tried to play the level three times now going through it various ways like destroying the pink crystal and then his core and the other way around I have mined literally every block and still nothing not sure if I am missing something sorry for the long sentence. the version I play is on steam I just bought the game today and was wondering if there is anything you guys could do to help I hope I'm asking the right place with this as well thanks for the help.