'P' I think of as the "Point and Shoot" mode (as a quick reminder, it toggles between gunsight and auto-tracking, it can take a few seconds to switch between the two).
In point-and-shoot mode don't forget to switch to cannon "c". I think of it as CP (combat pilot time), and hit my C and P keys. Mental jogs can help relax you in times of stress, also soothing music when available.
In this particular mission you have the advantage that the Mi24s come down the valley towards you, head-on aspect. Lock them up for a long range VKHR shot, zoom in and make the gate as small as possible (use the [] bracket keys), target the chopper a little above centre if you can. Not important but it can help if it turns.
You might also want to use the AA mode AND the AA/HO mode together if you have time to fumble for those controls.
I wouldn't recommend the HMS in these situations. It's hard enough to fly and point your head in the right direction. Keep it simple. Also try using the wingman as a human shield, tell him to engage the bandits to give you some time in getting your targets locked up, this is funnier in multiplayer situations.
Hope that helps a little.