Parnell needs to learn to use his jetpack quite well, because he has to be an extremely close range assault to pull out most of the damage. His rocket launcher is pretty weak, so it’s not a great idea to use it. However, you can use it between reloads and such, since your shotgun will auto reload while on the launcher. All in all, his rocket launcher has little to no use; much like Markov’s weapons. You can also end up killing yourself, if you use SS wrong. Or you could waste an extremely vital CD.
Use his CD efficiently, if you don’t, you won’t come close to the other assaults damage. Takes some practice, but you’ll get there.
When fighting, aim for the head/chest. Those areas with give the most damage potential, try to avoid limbs…
If you’re going to use shotgun, you must learn to avoid hitting the limbs… They will halve your damage.
Preferably, try to get the monster to attack you. If that happens, it drastically opens up the headshot potential.
Parnell is all about aim in the end… If you miss shots, you aren’t going to do well. His damage potential drops so fast, it’s crazy. This is a main reasons people don’t like him, and call him a weak assault. ~shrugs~
My favorite comp to run with Parnell, would be Laz, Cabot, and Griffin/Maggie. Griffin/Maggie both give some form of control. Griffin can be used to keep the monster off Laz… As well as this, Laz will typically be focused, so it gives you a general area to focus fire him in. When he pushes for the medic, you can openly push back without him disengaging much. I prefer Laz, because I play an aggressive Parnell playstyle. If I drop, I like having a decent Laz there to get me up again. But, I also like Val; due to the weak points/slows. Both of them help your damage output greatly.
With Parnell, run Capacity. It increases burst in SS, and it is great for out of CD as well, as you have so many more shots.
Another tip I could give you, is try playing Sunny a bit? When I played Sunny to begin with, it helped me learn to conserve jetpack much better… Boosting around teammates so much, you learn how to keep up quite quickly.