@Stella_x_Sin Like all the problematic weapons and specialists in this game, you need to alter your tactics to achieve the level ups.
For Lis, I hit T1 in a reasonable amount of time and found the rocket to be very useful in long distance standoffs with snipers as well.
For the vehicles, all electric trucks, Tuk Tuks, and ATVs are one shot kills. Keep your ears open and switch to the missile to chase them down. For others larger vehicles, if you see one that has damage at 50% or better, hit them for the credit kill. That said, it takes time for the missile to reach its target and it may hit after someone else's missile hits them, giving you the kill credit.
And the one gem is that if you see an enemy get killed out of or bails from a vehicle, hit the empty vehicle with your missile within about 5 seconds and you get credit for a vehicle destroyed.
Discarded, Kaleidescope, and Breakaway are great maps to situate yourself at vantage points and watch for vehicles running around.
Don't be dismayed, the AA rocket level up is a lesson in frustration and much harder.
