The player gravitating towards doing the same thing again and again isn't necessarily bad, if that thing is fun and doesn't break the game. It might be that you've just found that stunning is your local maximum of fun, and making it so stunning is more costly or less effective might make things less fun. Would the game still work if "stun" was the default action? (I don't know what your default action is -- attack? -- but you could try switching it around so that stunning is the default and attacking is always special.) Having something be the default can switch the perception of it being overused at the expense of other things.
Mmmh it is an interesting vision. I am not sure the stun skill is a fun, what i feel when i use it, is more like a tiny break in the fight.
The default action si the attack and the attack is also the only way to make enemies react and launch next turn.
Setting the stun as default would have no sense in the game. It will eradicate a big part of the game strategy because you will be able to never take counter-attacks.
I don't think it's desirable for skills to be equally useful and equally costly, just that the power/cost curves are reasonable, and that there's some point in the game where there are challenges that regularly exercise that skill. (So at the point where the player has mastered the "just use stun" gameplay, they're venturing off into a new area where just relying on stun won't win the battle anymore and they have to add new strategies to their play repertoire.)
I understand that, nevertheless it is more difficult with a procedural game like mine to make specific part of the game requiring to use these skills. And the risk of progression's irregularity can increase. Also, my goal with this game is to give to the player tools to face threats with mutiple approches, not any approaches but several.
It looks like you already have a complex elemental system of immunities, though, and adding addition immunities (like immunity to stun) might be information overload. (Could the skills simply piggyback on elements, so that lightning stuns, ice causes beasts to slide further, etc.?) Maybe stunning is implemented as a sonic blast that stuns the target but wakes any nearby sleeping beasts -- and that's not a problem in the early game, but as the game goes on you come across bigger herds of mostly-sleeping beasts and have to be careful not to wake too many of them up.
I thought a lot of time about what you said here, but i couldn't be able to find interesting passive skills that were equally useful or even coherent between each others. I put a lot of effort on game's harmony/consistency so i do my best to don't have thing that seems to be out of context. And stun, in a way makes me feel like this.
To be clear, in that case, it is like you have to run a marathon, but hen you you meet supporters giving water, one of them gives you a bike allowed on the run. That's what i call "out of context".
Recently i did a chart ith the skills to determine good balancing. For now it is in french, but if i have some time i will translate it to share it here.