Make good use of rechargeable grenades. I didn't know they recharge for free so I haven't bothered using them for like half a game lol. On Nightmare XD It gets a bit easier when you get Syphon grenades that drain enemy health and give it to you. Especially when you combine that with unlimited ammo skill, when health is above 100 HP. That helped a ton in latest levels when bunch of biggest ass enemies are being thrown at you.
Boss fights aren't all that hard by itself as they are tricky. Bosses aren't just bullet sponges, you have to figure them out.
Remember, best way to stay alive is to learn how specific enemies attack and what are their weak points. For those shielded bastards, aim for the gap in the shield or use grenades and lob them behind their shield. Explosive weapons also work well even with direct hits.
Boss fights aren't all that hard by itself as they are tricky. Bosses aren't just bullet sponges, you have to figure them out.
Remember, best way to stay alive is to learn how specific enemies attack and what are their weak points. For those shielded bastards, aim for the gap in the shield or use grenades and lob them behind their shield. Explosive weapons also work well even with direct hits.