In GURPS MECHA, how to aim under dogfighting? Dogfighting mechs moves
wildly, so it seems hard to aim, but fighters launch missiles, and even
shoot down enemy with automatic vulkan(sometimes), how to resolve that?
And, dogfighting "rounds" are 10 sec. rather than normal 1 second per
turn. If they can do aiming under dogfight, how many seconds they can aim?

Dogfighting combat is a simplified *cinematic* system that is radically
different from regular GURPS combat. There's no aiming or snap shots --
the direct-fire phase lets you shoot at no penalty for *not* aiming and
no bonus for aiming, using all the usual modifiers (range/speed, size,
neural interface, active targeting, targeting computer, etc.). The idea
is that you used *some* of your 10 seconds of maneuvering to get a bead
on your foe, but you weren't able to just sit there aiming.

Weapon Acc is mostly used for carefully placed shots in "realistic,"
wargames-style set-piece battles.

SP.