Class
I used engineer for being able to repair the core with scrap, RMB is ok too.
Edit: RMB refreshes your shields when you reach a certain level

Weapon
For pistol I'd say just use whatever yer comfortable with, however engi gets laser bonuses.
Edit: PPQ laser pistol is amazing, especially with full auto

Loadout
For the loadout slot I highly recommend Sseth suitcase turret due to its high hp, decent damage and being able to keep a spare one in-case you get in trouble. Its invaluable for helping defend the other side you aren't on.

Difficulty
Since difficulty doesn't affect unlocking I disabled everything except mag ejecting for the faster reload and muscle memory.

Map stage
Since you only need to beat one stage type I chose option 3 due to it being the most enclosed with decent cover. It also allows you to make great use of the Sseth turrets with their limited range.

Optional

Item selections
If you don't have riotguard class challenge regen reward then I highly recommend increasing healing crystal chance. Edit: (Healing crystal may have been nerfed since guide was made)

Unstable current is pretty useful for surprises or ones that may have snuck past.

Also give power icon boost to weapon upgrade kit.

Quick rundown of the basics

While there's no enemies attacking feel free to explore the map to get both its layout as well as see where all the vendors and upgrade stations are.

At the end of each round you can use the giant ammo box and the mystery crate near the core.

The big boss (which is usually the only big threat to the core), starts at wave 30 so make sure you keep the core topped up with yer overclock item and have a lot in reserve so that you can atleast get past the 50% health mark.

Strategy

Weapons to buy
Personally I only found the nailgun reliable due to its versatility and ammo regen however feel free to try any others if you can do well with em.

Items to buy
I recommend: Unstable current, Healing crystal, Extra drones/turrets and Hyper feed(if you have a lot of spare money. Its only really useful at wave 30 with nailgun). Edit: Healing crystal may have been nerfed.

You can get core upgrades but they tended to be meh for me, they are cheap tho.

Getting Sseth turret to 200% is useful.

Fighting
Use Sseth turrets to help stop the other side from being overwhelmed, its also useful as a decoy when yer shield goes down.

If yer overclock is isn't upgraded it has a max of 100 scrap so make sure to top up the core and missile turrets if its maxed. Its also surpisingly good for almost doubling Sseth turret lifespan.

The rest you can probs figure out if you've played before. Theres general guides on the main games steam community if yer a new player struggling. https://steamhost.cn/steamcommunity_com/app/528230/guides/

And don't forget to open the mystery chest at the end of each round and top up when running out of ammo.