That's funny, I just uploaded a proxy Hochfir gameplay video yesterday! Haha.

I'm not the best person to take advice from as I don't play her often, but I usually manage to do pretty good with her using a few simple tactics.

Try to strafe as much as possible. Moving in unpredictable patterns while shooting makes you way harder to hit, especially with proxy's small hitbox and fast movement speed.

It is ideal to sprint/long jump through open areas where you are easily visible to the enemy and very vulnerable.

Flanking is great as you can use your game sense to compensate for a lack aim, if you find aiming with smgs challenging. Catching unsuspecting enemies with their pants down can be very satisfying too. Make sure not to overdo it since you are an engineer in the first place, and should be helping your team out with the objective.

Know when to take on a fight and when to retreat. Proxy is fast and is a good candidate for the "hit and run" tactic, at least in casual servers. If you are low on health and there's a lot of time left before the next respawn, retreat and leave a gift around the corner for those that dare chasing you ;)

Anyway, best advice I can give is just play with her and experiment with new things, you'll be a scrub destroyer in no time, you get better with experience.

If you're interested in how I play her, feel free to check out my video https://youtu.be/u-UnfryGP-Q

Good luck man!