Also, just as you're about to start killing them with a shotgun, change tactics to running. You'll auto-dodge 1-2 plasma shots without having to do a thing, and the rest of the level is pretty simple, so save your run time for then.
Of course, unless you've gone for a MAD style build, you'll want to change from your shotgun to kill the shambler. Regenerating life, armour and shotguns don't mix.