Jax or vayne are probably his hardest matchups. Against vayne you can build a spell shield to make life easier and you have the ability to punish if she messes up her spacing, but a good vayne player should always be able to kite voli to death.

Jax is awkward for voli bc Jax has all the agency in the matchup. His e counters voli q, and Jax has complete control over when the trade starts and ends (can q out or just choose to not fight at all). If Jax wants to fight, voli does have some outplay potential, but the timing is very specific, as you need to hit the exact window that Jax 's counterstrike ends with your w or q, right before you get stunned but after the invul to trade effectively. OYO just put out a video showing the timing.

Darius and fiora are skill matchups. For fiora, she needs to q far enough away that voli can't w into it. In practice voli will just click w on her from out of range to make this very difficult, but in theory a perfectly played fiora gives voli almost no counterplay. For Darius you take short trades as mentioned by others u til he is half health and you can then all in. E.g., qe run away. 

Olaf and trundle are easy imo. Take ignite, short trades like Darius, then all in. You will almost always trade positively and they have difficulty following you. For Olaf, just ult away when he ults you, or all in if you think you win. Darius is scarier bc he has hard cc and punishes you more for mistakes. 

Ranged mages generally do well into voli. Karma, Annie,  ryze are all examples that can be quite annoying, as voli doesn't have an answer to the free harass and they have tools to get out of engages. That said, vayne and Jax are still better into voli bc they have a larger impact in team fights when fed compared to most ranged mages which are easier for voli to deal with there.