Create four (or however many) skills that are identical except for the element, and have the enemy choose randomly between them.
That's presuming this is for use by an enemy, not the player - the OP didn't say.
I'm trying to make a spell that does low power random elemental damage.
I Googled it and came across
this thread which has a kind of janky but functional solution:
"Make multiple instances of your skill, all with the same name and the same stats, but different elements. Create a Common Event that is triggered by each of these spells. Set this Common Event (that is activated every time after any of the variations are used) to have the character forget
all of the variations of the skill. Then have it choose one of the variations at random that the character learns. "
What that leaves out is that in order for the common event to know who is forgetting and learning the skill, you'll need to put a temporary state on the actor when they use the skill - have the common event look for the player in the party with that state, do the stuff it does above, then remove the state again.
So this is similar to what Aoi proposed, but with extra work to have it work for players.