That trap before the blue door is the evilest trap ever.
This is far the most common failure reason... 2 in 3 attempts end up like this if I reach this location. You finally managed to time that fucking door so green mechs won't fire in, then you lose a promising run, because the rocket mech dared to fire more than one.
Just knocking the door from inside with a rocket takes 45 shields. 2 rocket and you are practically dead.
The method is to destroy it is using the "shoot through door" glitch: If you push against the door while shooting you can fire though it. I don't use this glitch whenever possible but in this nightmarishly hard level this is the only way to maximize the chance that the mech fire only 1 rockets. If you are lucky and managed to fire through the door once, you can stun the mech for a moment so it won't finish its volley. After that the door is open enough to destroy it.
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Other: Today I almost managed to survive this level. But lost it at the very-very end... Only 10 days to go, then I give up.
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