Just random, synergy units have nothing to do with it, they just make the most of it when it does happen. I've done upwards of 40~ runs I'd guess and only seen them 5 times in various amounts (1x twice, 2x twice, 3x once). You'll know you have them when you go to the second stage of the level cause it'll say 2/4 instead of 2/3. They look like a cross of metal slimes and something outta Kirby. I've never played BF so no idea of their relevance.
XI Wiki



