Sorry for not responding sooner, but I have only just plucked up the courage to give it a go again. Long story short, the first Undercroft I got was of course survival, and thanks to your replies, I actually managed to survive! the best advice was to stay in one place and wait for a few life supports to drop before running forward to get them, but also waiting until 50% left to activate one of the pillars. I think it helped that I got given my Hek which could 1-2 shot most of the enemies! Although I was crippled with using Wukong who I have never used so had no idea what I was doing with him! I was given the choice of 3 that I own but have not used yet. Which actually really works against you alot, as they start at zero rank and have to be ranked up - so basically you start with just the one ability available. Anyway, luckily I did it with about 15% renaming! I didn't actually manage to reach stage 6 on that playthrough (failed the Void flood Undercroft), but you keep intrinsicts up to a certain point so that enabled me to upgrade some stuff.
So, next time through I was lucky to get Loki which has recently been my main and my Ortho which just ripped through everything - so I guess it is definitely the luck of the draw with Frames and Weapons! Anyway, I enjoyed this playthrough and did a few side quest to build up the Decrees and managed to reach the Wyrm (I got Excavation and Exterminate which were both a breeze with Loki and Orthos!) Sadly with just one section left, I ran out of revives :( - soooo close!) Not sure how you are supposed to deal with all that fire and all the enemies at once - and not a single Energy pickup in sight, so ran out of invisibility. Are there not meant to be Energy pickups or did I miss them?
Oh, and I always struggled a bit with Drifter combat, so decided to buy myself the Edun - what a beast that thing is, made everything so much easier.
Anyway, I really appreciate all the replies, they really did help. Thanks!