I happen to have been thinking about leo vs venom and looking at frame data because I'm working on commentary for my matches. If leo does what amounts to his rekka, ending with 236+h and you don't instant block then he has +1 frame advantage and you have no normals that can even become active fast enough to beat his brynhildr k, and you won't beat bryn~s either, since it's basically a disjointed hitbox and comes out in 6 frames. c.S is your fastest normal at 5 frames, so best case scenario without instant blocking is that c.S trades with bryn~s, which isn't really ideal. If you didn't instant block you could also attempt to throw depending on how close leo is, but bryn-k has throw invuln, so that's not that great of an option either. He can't cancel the stance without eating free attacks, so he's not going to throw you, so if you didn't instant block, then the best option is just to block low and look for high attacks, since you can just react to those. If he does that high-hitting special that staggers through your block if you block high, then the best option is to backdash. It's only active for one frame, so it should be easy to backdash.
If you do instant block, the best option is probably to just throw him or teleport out, since 236+H when instant blocked is -3, which is too small for your moves to become active, but if he's mashing he'll eat attacks for sure.Throwing will, depending on the combo you choose to do, knock him away from you or corner him. If you do c.S and you're too close, then he can throw you before the move is active and your hit box gets bigger when you do moves, so it's probably not technically correct, and he can just block. I'd argue that you want to stay half a screen away from leo, because his dash sucks really bad, so you can just zone the shit out of him if he's half a screen a way. There's really not a lot he can do about it. Poke a lot with 5S over stinger aims I think, because he's likely to try to IAD if you do anything on the ground, and you want him to commit before you 6P him. Try to leave some of the balls you summon unhit, so if he corners you, you can teleport out if you instant block the end of his pressure strings.
You basically win this match up if you have enough space to zone him, since his movement is pretty bad.
Edit: I just found out that I've been missing out on the majority of throw option-selects in guilty gear for a decade. The only one's I've been taking advantage of are burst option selects. With this in mind, I'm revising my suggestion for when you instant block leo's 236+H. You want to OS 6HS+S. If he's doing bryn~k, then he eats CH-c.S, and if he's doing almost anything else you throw him. It's probably the option venom is designed to use in this situation.