I've already moved past the phase where I quickly run out of ammo and get killed with no way to defend myself, now I'm facing difficulties when dealing with higher levels, which are actually dangerous for unprepared marines. Most of my games end on level 4.
So I was wondering, should I stay at lower levels until I scrape together enough XP to improve most of my skills, possibly getting a weapon still to master level?
I usually get to a state where the lower levels don't really pose too much danger to me, with plenty of ammo and medkits. I tend to climb the towers at that point, probably getting to dangerous places too soon. Should I, for example, fully explore all/most of the towers before going up? Should I spend time camping and just shooting aliens to get XP while I'm not pressed for ammo?
I tend to skip a lot of potential XP simply because I want to get into action. Besides, lack of enough keys/explosives prevent me from fully exploring millitary/security towers, the layouts of some towers (namely medical and civilian) and/or radiation usually make me explore only the remaining towers, unless I need to find a working elevator.
So... is there some rule of thumb how much XP I should spend before venturing upwards?