Mostly because there are big interconnected areas on the level where mechs can roam freely.
Fortunately it gives you lot of stuff.
You should start by killing the two spikes near the beginning. One can be from far away the other can be destroyed from the blind side, the next should be heavy driller 2, so you can get the plasma gun right away.
The lou guard tends to hide.
Many things can go wrong. Most often one of the spikes can rush forward causing tremendous damage.
Even after lot of practice, you must expect unexpected encounters in the main area. You can say you are safe only when you cleared every mech from the main area and finally got the blue key.
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