Weapons :
Use ak47 and glock. Buying armour (go left not right in the craft menu) also refills your ammo, and is important for rounds when enemies have guns.

Killing Bosses :
It's super easy, buy a land mine and just run into the boss. You'll take 100 or so damage, but you can't stuff it up and the boss dies ez. Just remember which waves the bosses come on, since you can put down and pick up mines you can just leave one around wherever you're playing in a place where a bot won't run into by mistake.

Turret Priority :
Turrets suck at level 1, so your first priority is to get the main room turret to level 2. Your next priority is to get the one of the two turrets outside to level 2, and then you need the main room turret to be level 3 (especially before the sl.ut wave). The two turrets in the cave are not needed, but you should prioritise the further away one since it gives you sound cues and will damage bots standing still shooting you long range.

Stacking Health :
Since buying +50 health heals you, you never need health shots. The important waves to stack HP beforehand on are 6 (the first octopus wave), the sl.ut wave (16?), and the raider boss waves (20ish?). You only really need 1000HP but nothing is stopping you from easily getting around 3000 before the game ends.

Anyway, once you've got the hang of instakilling bosses, the rest is pretty simple: let the turrets kill everything for you.