That is called Adult arachnoid. You should have a shotgun but it takes quite a few shots to kill it. What difficulty are you playing on?
You should be able to kill it, without loosing too much hp. On Serious I killed it, losing very little. Just hide behind walls, learn which walls are destructible and which are not. Always keep a wall between and Arachnoid when it is shooting you. You should be able to shoot it 2-3 times and than hide, you will see it raise its hands with machineguns, and that means bad... (time to hide).
Hint, there is a laser gun secret earlier that level. But it only has 20 ammo, unless you found more laser ammo in secrets during 2 levels. Might not be enough to kill it, but can help, as it does more damage than shotgun and quickly.