All three domination spells' descriptions (Dominate Monster, Dominate Person, Dominate Beast) contain the following paragraph:
You can use this telepathic link to issue commands to the creature while you are conscious (no action required), which it does its best to obey. You can specify a simple and general course of action, such as "Attack that creature", "Run over there", or "Fetch that object". If the creature completes the order and doesn’t receive further direction from you, it defends and preserves itself to the best of its ability.
What would be considered a simple course of action?
For example, could I tell a dominated humanoid wielding both a longsword and a longbow to attack using a specific weapon, such as "shoot the ogre with your longbow" instead of just "attack the ogre"?
Or, another example, would it be possible to command a dominated dragon to breathe fire onto his enemies, or is that too specific? After all, "breathe fire on the zombies" is not a complicated course of action, is it?
The reason I'm asking is because commanding the controlled target to use a specific ability - which might or might not be at-will - seems significantly more powerful than just "attack that guy". Still, I think it could easily count as a "simple course of action" - but is it "general" enough?
That the course of action requires the use of a one-time ability doesn't matter - afterwards, the creature will have "complete[d] the order" and simply defend itself. Also, you can just change the command as you please on your next turn, after the ability has been used, since changing it doesn't require an action.
Primarily, this would be a matter of balancing. Many monsters have semi-strong at-will abilities/attacks (that would most likely be used when told to "attack a target"), and a few stronger, one-time or rechargeable abilities/attacks.
Please note that this question is not about what happens when you use your action to take full control of the creature, nor are answers in the line of "depends on your DM" helpful - I'm the DM, and I'd like some guidance on the matter because I'm unsure myself.