Kieran seems to be a lucky charm for me. He's saved my ass plenty of times. One of them being that level with Valtome (4-2, I think). Valtome almost always hits one of my units whenever I go to attack him. With Tibarn, it's not really a huge deal, he can kill Valtome in one hit, but I don't like using Tibarn very much in that chapter because I prefer to level up my other units. I like attacking with Kieran, Ranulf and Elincia for the funny conversations.
On this one chapter, I had one character next to Valtome who was clobbered pretty good. If Valtome went after him on the next turn, he'd die in one hit. Thankfully, I wiped out most of the other enemies within range. I went in to attack with Kieran. Valtome missed, Kieran activated Sol. It was beautiful.
As for epic fails, it's hard to narrow it down.
Pretty much all of 3-1, the entire chapter, ended up being fail for me on my first playthrough. I had to reset so many times. Now I have no troubles with it. But my most memorable fail on that chapter went like this:
Me: Crap. I hate Shinon. He's a jerk. I don't want to use him.
Shinon: -criticals an enemy-
Me: 0.0 Maybe I will start using him...
Shinon: -wipes out most of the enemies on the map without taking any damage-
Me: 0________0 I shall now worship him.
Shinon: -gets critical'd and dies whining about how lame his death was-
Me: D= D= D=