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5 G

Best attempted on easy, at the beginning of the mission move all your units towards the back of your base and refrain from building any units as your turrets should be able to deal with anything counter attacking once upgraded, now the tedious process of repeatedly mac blasting the scarab begins if you have the emperor skull collected from the final mission activated this reduces the mac regeneration time in half, otherwise it will take about 20 minutes to destroy.

As the Scarab becomes damaged it may start randomly destroying cover blocks including the ones protecting your base, however don't panic as in my experiences if you sit tight it tends to destroy the front block and move on.