Maybe im not as rubbish as i thought
I did Lex challenging last night, i died once in the offices near the beginning because i wasnt paying attention, but cleared the roof without going down. Managed to clear the end section after 3 attempts. Barrett was easy enough, one clip (G36C) after a water boost. stats are something like 330k/400k/120k AB build.
Yeah, just kept getting caught in the wrong place, hit by the sniper or attracted both heavies at the same time. Clearly not used to the change to the seekers and turret though.
Some times things just don't go your way, maybe last night was one of those!

Cheers