This isn't that hard. Some tips:
Don't get too close to a grunt or he'll one-hit melee kill you. If you do get close, punch him! Throwing grunts or things at grunts will save ammo and is more fun and sort of the whole point of this game.
Your pistol can take down most enemies from far away relatively easy, including Polycraft and BioMechs that are looking the other way, and there's ammo everywhere. Use it a lot!
Speaking of BioMechs, try to punch/throw anything you can find at them when they're far away, a lot of them will stupidly sit on a ledge and take the abuse without doing much in retaliation. Sometimes you can get them stuck or confused by standing behind certain objects or on cliffs at certain angles, try to find these spots if you're having trouble dispatching any of them. If there's more than one in close proximity, use grenades to take them both out quickly.
Snipers will peg you in one shot, keep swinging when you're in their sites until you're in cover. They're more dangerous than a herd of Polycraft, too, so they're priority number one in all combat scenarios. If you can get close to them, that's the best option, but if not, you can usually counter-snipe with the pistol from behind cover from upwards of 150 yards or so.
You can also skip sections that don't have doors so go ahead and swing on through if you like.
You can equip and use that blue cannon thing that some of the BioMechs later in the game have to shred up armies of grunts, and sometimes the game will save that you have it if you get to a checkpoint with one, but some checkpoints don't, I'm not sure why.
The boss at 75% progress is probably the hardest part, I've posted instruction to kill it on the "Can You Dig It" achievement.