I'll send in smaller groups of more powerful characters later on so that sounds like the perfect time to have your assassin group. Maybe my characters are just about to finish off the other characters but then the assassins sneak in and kill them just in time to save the rest of your group.

You can do an entry post any time you're ready. My characters haven't entered the area yet until everyone gets an intro post so you can't really attack them yet. Oh yes, and for one more important thing, someone already lays claim to fighting my grasshopper character,