So the problem is I have the game saved at Legate Lanius' camp. The problem is you can't fast-travel or leave the Hoover Dam level to stock up on supplies & weapons or finish missions to build your XP up.

I'm low on stimpaks and aid in general, I have a lot of ammo but perhaps. And I'm only like a level 15 or 16.

I'd probably be higher if some of the missions I could complete, but the game is so damn bugged that it actually prevented me from completing a couple missions. For example, Camp Golf where you have to arrest the Chief. His office door just closes and nothing happens. I've seen others on the internet mention this (I'm on Xbox).

Are there places around Hoover Dam to stock up on weapons and aid? Are there cheap & easy ways to defeat legate, like standing on a high object where they can't reach you and just picking them off?

Also, why are like 8 guards after me, I thought there was only supposed to be two. Is that another glitch? Because I killed a bunch of guards before fighting Legate.

Sorry dude, but I think you're screwed. I got myself in the same boat, not out of bugs though but just accidentally rushing it. I couldn't really find a good place to snipe the Legate either, but I'm sure they exist. If you have a silenced weapon your best bet is to try pick off the legion as you find them, be careful though as they'll be in tents, around corners etc. If you don't have a silenced weapon then just do what you can from a distance and make sure you can run backwards lots, as legion tend to be unarmed so can be easily shot down if you have space behind you. There is also a small amount of loot and some aid in the lockers found in the tents, just small stuff like healing powder though, but can also yield some ammo and guns. If you haven't noticed, NPC's can't jump, so it'll be handy to leap up some place high.
Lanius is going to be a real pain in the butt. If you make enough noise, he comes running. All I can say is good luck.

This post is probably way too late and you've no doubt finished the game by now, so I hope all went well for you XD