Whether you like him or not, buying characters in this game with cash is a big waste of money. Dio is especially a waste, and the worst AP character imo.
Just keep playing him and unlock all of the OP skills.
A nice Dusk Bringer's Hybrid skill build for dungeon - Grand Chase
Hybrid skill guide that goes from 1st job to 4th.
Going hybrid with Dio works better than sticking to a single side in dungeons. Probably more fun too.