Veteran Exe Stance Infantry Lasgun
 Veteran Build - Warhammer 40k: Darktide 

Shovel is for bonking melee enemies to get away, hence decimator and hammerblow.

Consider swapping the ranged strength from marksman for the krak damage on grenade tinkerer for better burst boss damage contribution.

When you're more comfortable surviving, swap a toughness curio for more stamina to maintain deadshot longer even if you miss a bunch in a row and sustain higher damage and quicker gunner clear rates over the course of a map.

If you routinely find yourself getting engaged in melee or taking chip damage because you're getting hit whenever you do, consider swapping to shredder grenades and dropping one when you're about to have to fight any sizeable horde.

As for actual strategy for survival - first and most important thing is to learn spawn points and objectives so you know where enemies will be before they arrive. Best way to not get surrounded is give yourself time to react.

After that it's about putting yourself somewhere so that your team mates engage things in melee just by virtue of where they are in relation to the enemy, rather than relying on them to come and help you out. As in if your team mates are on your left and in front, and there's a bare wall to your right, stick to that wall and check behind you so that your ass is covered. That's more often than not the most important part, not necessarily getting a lot of fire down range.

The third thing is trying to position yourself so that enemies have to round a corner and bunch up to get to you. Yes that is the same thing as saying take cover. It means that if there's something between you and enemies, you have extra time before they reach you, their movement is more predictable and headshots are easier, and having a choke point like a door, set of stairs, a couple of boxes etc. means you can target the most dense part of the pack and fire there to achieve higher effective fire.

When you take melee contact, first and foremost try to run away, but if that's not possible it can often be more important to finish killing a bomber/flamer first while dodging and then swap to your melee weapon. When you do, spam heavy attacks and pushes to keep yourself from taking damage, and the moment you can, try and get back to your team.