Marauders are small and hard hitting aliens, they are hardly ever found in small groups, they can easily outmatch sub-elite ships and BigBoys and Leonovs are no match for them. The best way to defeat them is to be in a strong group and keep moving. They can be found in the Epsilon Gate and in the group gate.
Marauders are currently being investigated, more info to be added. FE Leonovs have a good chance of facing this alien.