Does anybody have the same gripe as me? The level the Gonarch's lair. Alright so I waste all of my ammo just to kill this thing. Then. Interloper which is long and hard. I find myself losing more ammo which won't help with the Nihilanth! What were they thinking! If anyone can help please feel free to speak up!

Quite a few people think the game becomes sub-par once you reach Xen.

I, for one, have no idea what they're talking about.

You didn't use an RPG on Gonarch? It's really the best weapon to use against the bitch. I always try to save some RPGs. The Interloper has a little bit of supplies in it if you know where to look but it's one of the tougher portions of the game for sure. Also probably my least favorite, or up there.
http://web.ukonline.co.uk/aghl-faq/walkthrough/interloper.htm     
 
Did a quick web search and found this. Summarizes exactly what to do.

If I remember correctly, I too was frustrated to the point that I looked up an invulnerability cheat just so that I could see the ending cutscene.

I remember being a little tired of the game by the time I got to Xen.  I was ready to beat it, and the game had continually led me to believe that my goal was in the lamda complex.  It was of course, but once I reached the Lamda complex the game was nowhere near over.  There were certain frustrations in Xen as well of course, the Gonarch being one (I thought it must have been the end boss, again, fooled me)
 
At least I can say I beat it without using a guide.  Oh except I had to use one to beat the Nihilanth.....  yeah Xen was definitely hard by todays standards.