'Rum'-pumped crabs are Braindeath Island crabs that have been mutated by having Braindeath 'rum' injected into them by Captain Donnie's General malpractioners.
They are aggressive to players of any combat level and you will be attacked by several at once, therefore it is recommended to use Protect from/Deflect Melee to reduce damage. Note that will also be attacked by aggressive General malpractitioners in the dungeon who are capable of poisoning you with their thrown syringes and therefore it is advised to bring Super antipoison with you. Furthermore, they will occasionally throw a syringe at the crabs and increase their attack speed. However, there are several spots in the dungeon where you will be safe from the malpractitioners including the room east of the entrance.
Brackish blades and Brine sabers have increased damage and accuracy against these crabs, however due to their weakness, it may be more effective to stick with Air spells.