Get used to imps, they are the most common of all monsters...

I will put up this strategy just incase anyone needs it to deal with imps, taking care of them first will minimise damage greatly i think, but anyways here it is:

Best way i found out so far to avoid its leap or jump attack is to backup or walk backwards, since the leep is based on a fixed distance, strafing won't work or rather i failed most of the time trying to strafe the jump. 1 shotgun shell close up can kill it(chainsaw works too), aimed at the head though, most usually 2 shells in the torso, sometimes they spawn at a distance, most encounters when fighting at a distance happen when there is some sort of barrier or gap that doesn't allow you to get close up, the pistol ain't bad at taking 1 imp out since its accurate, but if your fighting several at a distance use a machinegun.

One important and useful thing i found out, and its especially useful when fighting those surprise attacks in doom 3 is to bind the flashlight to the right mouse button, as someone suggested above, since i'm pretty easy to freak out, i get frustrated finding that F key ~:p right now i have a much easier time killing monsters and less frustration, oh and also cuase less damage to the keyboard ~:p