True enough I haven't gotten the chance to but normally I make my villains end game strong from the start to make sure they last that long but if by some miracle the players manage to get to him or her, unless absolutely it's going to end up ending the rp early, I let them die. In that case I'd pick an earlier villain increase them to major villain status and let that person take their place. However I do my best to keep my villains alive sometimes doing rather ridiculous things but in all fairness if they died well then the whole plot of the rp unravels and I can't continue. If someone wants to try to change something in my roleplays or wants to do something differently I at the very least try to consider it but most of the time it can't fit into what I have planned and I can't explain without ruining half the surprises I have in store about my roleplay.