I am stuck on this task - only have 10/20 and have been stuck here for 2 days - any formation help please

Use any formation and keep going for however long in any chapter. After a while, go back to maybe area 10 or whatever, remove all crusaders, and level up Bush Whacker for health. Put him in front, level up Biff to get all his stuff, place Dark Gryphon behind Whacker, and start testing around to see which area having Biff in your party allows him to *SLOWLY* damage enemies. Once you find an area where he can minimally harm enemies, take him out and wait, like, a minute or so. Let loooooooooots of enemies pile up on Bush Whacker, and when there's a huge crowd (say, 25+), put Biff in your party and his carrot should kill them all within a few seconds.