Equipped creature gets +2/+1.
Whenever a creature enters the battlefield under your control, attach Ronin Warclub to that creature.
Equip {5} ({5}: Attach to target creature you control. Equip only as a sorcery.)
The triggered ability can attach Ronin Warclub to an untargetable creature because the ability doesn’t target.
The triggered ability won’t attach Ronin Warclub to a creature that can’t be equipped (such as Goblin Brawler).