First, the game doesn't recommend 12 000 power, but 12 000 outpost value in credits.
Power (not credit worth) boosts turret damage at the rate of +1% damage for 10 power until you reach 3000 power, then you get diminishing returns until the buff maxes out at 10 000 power for +500% damage, which you only reach after grinding a lot after the end of the campaign.

Anyway, "don't kill fast enough" and "keep running out of ammo" means you have an efficiency problem. Increasing power by adding small generators will improve your efficiency. Put them on pillars for denser packing (3 pillars can hold 8 small generators).

You'll have to grind the previous zone to build generators, storage crates (more materials in stores=more ammo reserves), and more ammo makers.

I recommend you vastly exceed (double) what the game recommends for a mission. I vaguely remember a mission where it recommended having 1 prewar artillery, I had 2 and still had a close call.

I don't remember what stuff I had available when I entered the snow area, I assume you have MG and basic (purple) 30mms, but what are the most advanced stuff you have researched?

The basic 30mm autocannon has one of the worst of ammo efficiency in the game. Upgrade them to twin 30mm or viper ASAP.
All 30mm cannons have an AoE so small it might aswell not exist, so they suck at crowd control. Use the "adjust" tab in the outpost editor to tell your 30mms to ignore scorpions (small enemies) and focus on medium & large enemies.

Against unarmored enemies, machineguns are 3x more efficient than the basic 30mm, so they may still be relevant at your stage.
"Bottom" machineguns have 20% higher DPS than standard ones, but the same efficiency.

If you have access to high energy matter (orange crafting material), consider cryo towers and mortars.
Until very late game, a group of cryo towers focused on scorpions will freeze them before they reach melee range, allowing you to basically ignore them. They have a conic AoE so they are best placed at ground level. Don't forget to increase their range in adjust mode.

Mortars have 2.5x the blast radius of 30mms, this allows hitting tight groups of scorps, but since they do kinetic damage (not pyro) they perform poorly against armored targets, so tell them to focus on small enemies, perhaps mediums when there is nothing else.

And then there is your operative.
-The rocket barrage of the mammoth squad should be quite good at your stage. I've had bad experiences with the passive that converts it to fire damage (or one of its upgrades - it fired only 2 rockets), but the electro conversion is good vs groups.
-For XENs, the "gale" firemode is very good vs groups even lategame, if you don't have it, you can use the "pinpoint explosion" firemode: shoot the front scorp and watch them all walk into their death.
-If you have the "battering ram" XEN booster, this boost damage done to wrecks and trees, and wrecks/trees destroyed by shooting them give materials as if you had harvested them. That means ammo for your outpost.

Using ammo makers in FPS mode is too slow, assign them a group in adjust mode (dbl click a maker, ctrl+1-9), select the group in command mode by pressing its number, and press shift+Z to instantly tell the whole group to work.