Well the last time I played I just went to the 2 houses that I needed to go to and didn't bother to kill the Titans.
I did however try to kill them in previous games and I think I used Meteor Strike (is that in this game?) wish I could remember for sure but after so many games after this one its not easy to remember what you used back then.
They are very hard to get rid of and you have to try everything you can think of.
Usually Fireball or Dragon Breath or Paralize or implosion work pretty good.
One of these will do the trick.
Don't remember if this game has shrink but that might do them in also.
I do remember that Fireball does hurt your own guys in these games so you have to be careful where you use it.
I hope that I was of some help.