You get Pelleas on your second playthrough. There's a 3rd option, so you can choose not to kill him at all.
As for the Tibarn chapter, on my Normal playthrough, I managed to not put a single unit I intended to use on his team (not a clue why). All I had was Elincia in the spot Volke spawns, and Volke blocking the entrance to it. If Volke was hurt badly, Elincia healed. If he was fine, she went out and killed. After a while, she was level 20 (amazingly enough), and joined Tibarn in killing the enemies. That was awesome (Tibarn pretty much solo'd the entire level).
On my first Hard (note: I didn't know of Ike's memory scene nor Pelleas, so I didn't start on the Clear file), I had a decent team (still the weakest of my 3), and I just gave everyone vulnearies and charged into them. (One group goes straight for him, blocking there for a while, while the big group charges up the right side. The right group splits up, one going North and one West.
On my hard just a week or two ago, my best team was in that group (well, not really. I used like 35 characters this time, and they were all equally level. But both the Paragons were in this group, as well as the Blossom, so people ended up the best here. I didn't even need to use the ones I used here in E-1 and E-2. Anyway, I just rushed around and killed all the enemies. There's plenty of terrain spots there, so place your mages there, and rescue them with Tibarn/Elincia if they die. Pelleas is a bit of a trouble maker though, so you may want to keep him with an untransformed Tibarn decoy with Vulnearies.
(Note: It may sound like I made it on my first attempt every time here, but I didn't - not at all).
All the methods are completely awesome, and makes it to one of the best Fire Emblem chapters ever for me. If you have a save at that chapter, I recommened using it.