You have pretty much perfect DoT team.
If you have good idol, you can try to swap plague mogul with Idol, I think he had better dot dps, just don't throw spear.
Also, It could be that you killing zombie too fast and lose time on ressurection animations.
Try not to spam abilites - just place dots on zombie with dracul and cycplops (use both ult at once when both are ready) and wait they eat Invokus.
If is doesnt help, then you have to wait to have stronger heroes I guess, can't advice more from that.
C1 is really lacking damage compared to C4 afterall.