This is a direct copy-paste from the move description:
Substitute does not block Attract, Destiny Bond, Disable, Encore, Foresight, Guard Swap, Haze, Heart Swap, Imprison, Miracle Eye, Odor Sleuth, Perish Song, Power Swap, Psych Up, Roar, Role Play, Skill Swap, Spite, Taunt, Torment, Transform, or Whirlwind.
A "smart" Breloom uses Sub first, so you switch in right after they sub. They can Leech Seed, Spore, Focus Punch, Seed Bomb, or Stone Edge you in this position. Any faster user of Encore wins right there. A Whirlwind user with Resttalk like Gyarados would work, but I am having a hard time thinking of one that can take a Stone Edge AND Focus Punch AND Seed Bomb.
Synchronize might work behind the sub, and it would ease prediction as to what he is packing. A sleeping Breloom behind a sub is just as dead as anything.