as far as i can tell, mind control cost no action points at all, and have no ways to be broken except for killing an enemy that is tougher than most of the regular infantry.
I have a few suggestions of things to remove mind control:
disabling the head
inflicting daze
using the paralyzer(or something like it) on the afflicted unit
armour/helmet attachment.
using mind control on the mind controlled unit, or the mind controlling unit.
inflicting panic on the mind controlling unit.
another suggestion is that war cry, daze and panic effects on mind controlled units also affects the mind controller, and other units the mind controller may be controlling.
because they have a lot of health and armour, and you need more than one sniper shot to hit the head, and they may not be in a line of fire.
They can cover a lot of ground very quickly, so it will usually need to be at long range to prevent them from using the ability at all.
using an assault to run up close enough to destroy its head usually guarantees that he will die on the alien turn
It’s only a problem if you’re facing multiple sirens at once, and that usually only happens if you advance incautiously into an area where you have no good LOS.
After you face one or two and figure them out, they’re not that difficult. As mentioned, they can only Mind Control your soldiers if they have more WP than your soldiers (WP cost of Mind Control = remaining WP of the target), and if you disable their head they lose all their WP so they can’t Mind Control anyone any more (potentially, unless one of your soldiers has 0 WP; I haven’t tested that). Also, Mind Control costs 2 WP per turn per unit to maintain, so if you disable their head after they Mind Control one of your soldiers you get your soldier(s) back on your next turn.
Just have your sniper shoot them in the head a few times before they get close to your other soldiers, or have your soldiers focus fire on their head if they manage to get in close before your sniper can take their head out themselves. Sirens are meant to be dangerous enemies if they get in close, if they were any easier to deal with they wouldn’t be dangerous since Mind Control is their main threat.
You can drain the sirens wp now, which gives you some defence, but I believe the distance at which the siren can MC is to great, and is effecting the games fun-factor. If they reduced it by two tiles I think that would be perfect and in-deed add to the fun-factor.
Sirens have definitely been toned down in the latest patch (being able to disable their head, stopping them from MCing) and are now quite manageable imo - I believe they fully lose the ability now, so even if their WP goes back up again, they cannot MC
I think they’re programmed to only MC one unit per turn (at least I’ve had them in situations where they could have MCed a few of my soldiers at once, but they only went for 1) - which could be a suitable limit for your priests too. I enjoy when Chirons fire worms at my priest. Lower their WP, then control ALL of them for free, and detonate them somewhere lol