{T}: Put a fuse counter on target creature.
At the beginning of your upkeep, put a fuse counter on each creature with a fuse counter on it.
Whenever a creature has four or more fuse counters on it, remove all fuse counters from it and destroy it. That creature deals 4 damage to its controller.
If the creature regenerates, the fuse counters are still removed and the four damage is still dealt.
If there are two Bomb Squads on the battlefield when a creature ends up with 4 or more fuse counters, both Bomb Squad abilities will trigger causing 4 damage each even though the first to resolve will destroy the creature.