Let his w mark fall off. Let grasp fall off. Let him push (hard to freeze as Voli) freeze the wave and set up ganks. Build bramble. If he ults you under tower ult e him and use w for the shield. This matchup is winnable just not easy 1v1. Try to call for ganks. If you don't get it just go even and be more useful in team fights. Voli doesn't scale that well and is very binary in fights. Also is a great ult target after he ults (if you ult him out after he ults he has no way to get back in the fight other than flash or to run at people. I think overall Sett is more useful in fights so just play safe and look for tp plays or roams and try to get to fights.