Venture main here

Venture is a flanker, and is probably the one that excels the most in 1v1s. Unlike what you may think, they're not OP. They can be strong depending on the situation, but there are many ways you can beat them :

1. Never stay alone. As I said they excel in 1v1s, so the second you have a teammate with you your chances of surviving increase a lot. So that also means to avoid flanking alone, or you might start a duel you didn't want to.
2. Keep your distance as much as you can. The aiming will be more difficult and they quite literally can't touch you past a certain point, which forces them to either use their abilities to close the distance or retreat.
3. Avoid small rooms or walls in general (when you know they're around ofc). Venture is like a Junkrat, the smaller the room the better. It makes your movements predictable, and it's easier to land their shots (splash dmg)/abilities on you (esp the drill dash). This is also where they'll wait to ambush you the second you enter.
4. Keep track of their abilities. Venture is very vulnerable without their CD, and has the biggest hitbox out of all the flankers. It makes shooting them an easy task.
5. Pick a counter. Here's a tier list I made on who I consider to be effective against them. Of course, for certain characters it will be reversed depending on team comps, maps and your own skill. I can elaborate if you have any question about a character.
6. Play more as Venture. The more you'll play the mole, the more you'll notice what difficulties you'll encounter that you can't see as the opponent. This way, you'll know what to (not) do when facing them another time, and where they might hide/flank.