Crawlers are difficult to hit with melee weapons r.r gotta crouch for that unless you hit them while they're mid jump, their hitbox is very small and close to the ground, so yeah. And the fence you basically have to crouch jump like in most games Space plus whatever you have set to crouch / duck. I have mine set to ctrl like in most fps.

The AA12 is a decent shotgun, though I still prefer the hunting shotgun myself as a big / large swathe of enemies choice for my secondary big weapon alongside the pump. Alt fire makes it a very powerful weapon against scrakes / fps. Even on a six man server on normal you can down a scrake with a DB shot by jumping and shooting it in the head with an alt fire DB shot.

One point of welding is to choke enemies in a hold point, like weld one door, all the enemies will be stuck behind it but eventually trickle through another area slowly one by one making a game easier typically. And the second point of of welding is for the exp points, because, support specialist needs welding points to be able to level, no matter how much shotgun damage you do you still need the weld points to eventually level up. You did better this game though, beginner or not, you were more aware in game and getting at the enemies quicker. :D