It depends. Corner shooting is always good, but usually you are spawned in a place without cover, so use a homing phase to position yourself near the exit to run when you need to. Then, don't go round charging for the Arachnotrons -- diagonal move around until you can either see or hear or get shot at by one of those things (you should have dodged the shot due to your diagonal movements). Fire at will until you get sick of it or something. If you have shotgun-type weapons, activate running mode and diagonal move like crazy as you shoot/reload. If you have Red Armour, wear it as it helps (full red gives 4 damage reduction, but since it is plasma damage, you reduce damage by 2 instead). Attacking Arachnotron caves without armour is possible but you will most definitely take a lot of damage in the process.
Also, the arbitrary cut-off I use for "will I survive another round without a medpack" is about 30%, which translates to about 15HP without Iro. 22% will most definitely need a medpacl of some sort.