You should be using practice, even if it sounds easy. I think this mastery test is a good lesson for that :p I got stuck on this test for a good while (had exp for MR13 before completing MR9), so I literally went and built stuff for it (banshee, redeemer), but the biggest thing with this one is patience. Every MR (MR15) test I've done after this one was ridiculously easy though so I feel like they should probably alter this test, especially since stealth isn't important in the game except in very niche instances. Edit: Actually I think I ended up succeeding by using the xoris so I could hit them from far away.