The best way to take down a Monolith is either:
1: Luck
2: A very, very, very large gun (read rail or D-cannon and a Vibro cannon is also annoying)
3: and the best one......Gauss, Necrons can take down a Monolith with their basic Troopers
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"I could calculate your chance of survival, but you won't like it."
"Shut up! Sit your ass down in that chair and DRINK YOUR GODDAMN TEA!" - Cid FFVII