1 and 2 I have already created a system for. Just go to a profile and search my posts, you should find it.
As for 3, you could use Plys mod and some math to determine where the enemy is facing relative to where you are facing, and where he is, you could multiply damage.
For 4, (with Ply's) you could attach a camera to the enemy, hide the players weapon, attach a character to the players cam, and animate it to show the attack.
3 and 4 are complicated, but definatley possible.
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