Yep, under the nose is a very good presure point. Imagining it applied with a knife blade brings tears to my eyes...
And the stomp to the arch, which is what I wanted to address, but Dwight covered that one pretty well. But be careful when lifting a foot to stomp or even step when struggling as that leaves you with only a half support - be quick and vicious - your balance doesn't need the weakening.
Rake to the eyes is great if you're facing the attacker. Really good if you can get to some keys in your off pocket.
But if he's behind you, use that off elbow hard and often! Center mass - sternum area if you can't get his head, groin if it's in reach, or his arms if you can reach 'em, he'll let go if you break one. Strong, vicious...
Should also mention the don'ts. Don't try to match strengths - don't wrestle around with the guy trying to show you are strong enough he can't force the gun out. Get some kind of equalizer - a weapon, or go for one of the soft spots on the guy. He's trying to kill you - there's no such thing as fighting fair is such an instance.
Don't give up.
Don't fight silently. Cuss, scream, call for help, threaten, whatever. Vocalization attracts attention and can be demoralizing to an attacker.
Don't stay near when you do break free - create distance.
Don't try to draw the weapon on your own until you have a safe distance from retention threats.