i got the same problem as the post above, everytime i try to attack an mercenary unit hired by my opponent it looks like this
can't kill them with anything else either, ranged attacks with no cap won't allow me to target them and spells that can kill units won't do anything to them, the only way i was able to do something to them is by Using Archeon's Roar ability to move them away from the city, but if the unit is inmune to fear then that won't work, at this point only way i have to get rid of them is to delete them with world editor
notice how they all have 0 life, this was also happening when i recruited mercenarys, i couldn't move them from the city they appeared in, couldn't heal them or anything, nor could they attack, but they were able to cast spells, so i didn't use mercenarys the whole game, i tried using them again the same turn i took the screenshot, they appeared to be able to move and had full life and their strenght value was displaying well, but then as soon as i gave them any promotion, they broke again and where unkillable but unable to do anything (the AI can still move the merc even if they are in this state, tho not anything else)
only thing i have thinkered with is the gamespeedinfo.xml in the mod directory to make the game much faster and the leaderheadinfo xml in order to replace Organized trait from Decius (who im playing as) with Charismatic, im playing as the bannor and so far everyother civ has the exact same issue with mercs, i do recall killing some mercs from other Civs normally so not all of them were bugged, tho most of them are.
it seems that as soon as they get a promotion by normal means they break, also something i realised aswell is that if you use an damaging spell or any sort of non combat damage to soften up a unit, on an enemy mercenary that seems to be working, they break and go back to 0.0 strenght and unable to attack or be attacked/killed, realised this when some mercenary knights where in my territory and i used maelstrom before attacking them only to see that they broke and i couldn't attack them no more
Using SVN revision 319, the lastest one
here's the savegame in case you need it