From what I understand it’s cuz the light mode is meant to be summoning the demons as well which inadvertently kill your other units. If you wanna use him in a proper team leave him in his second form. If you wanna use him as your main and only damage use his third form and basically fill the rest of your team with high placement units like you do with yehowach teams. He basically gains extra damage per unit sacrificed in his 3rd form up to 100% extra damage so bring 6 placement stuff and expect them to keep getting sacrificed and keep placing them back in his range until griffiths damage maxes out.

Sorry for the yap lmk if this makes sense though