Game breaking build?
Fill your entire roster out with foot rifle infantry, broken into squads, in a BV2 game.
Why?
Because if they have four 'Mechs, you have at least forty units, if not MORE. Resolving your turn might take an hour, if you're lucky, and you'll be there for multiple days as you try to see if the 'Mechs slip on the bloody corpses all around them or if the rifle infantry plink enough damage and TACs to kill them all.
THAT breaks the game.