How is the target calculated in Mir anyways? Isn't it non-alpha parts of the object are then set to be a collider, i'm not sure.
Best way would be to integrate an easy way for Taoist to target players and an easy way for all players to target mobs (by nearest). I know "Tab" is used for item pickup, but in alot of games "Tab" would be to select the nearest enemy. The other part would be the FKeys, these generally allow you to select players sequentially (f1 = Group Member 1, F2 = Group Member 2 and so on...) However FKeys are in use, so that would not work!... Maybe you can add a Alt+Number function for Player selection (if in group) and Alt+` to select nearest enemies.
Also you could add a check code to poison to NOT poison anything with less than half its duration (so if poison lasts for 30 seconds and it still has 17 seconds left = Can't re-Poison, but at say 14 seconds, you can).
Not sure if this is any help but that's how most games sort out the targeting issues