First off, you can't get any MK weapons until Fargabaad, which is over some mountains, which only the Invinclible can cross. There are two caves with spliting enemies, the Underwater Cave, and one in Fargabaad. the underwater cave is required to pass through to get the Invincible. The best way to get through is to use your classes with the highest skills, or if you have a Black Mage, blast them away with whatever is your highest spell. (Thieves are recommended for their high escape rate)

If you defeat them, you will find they are not very efficient EXP givers, unless you make them multiply. A nice and fun way is to stick everyone in the front row, and wait for an ambush, and have your white mage use Exit on all but one splitter, and attack. If there's only one splitter left, and is low on EXP, and you wish to get EXP, heal it, and resume attacking.