I've just beaten Fire Emblem: Awakening on Normal Difficulty

It's actually quite easy on normal, mostly because of the ease of level grinding in this mode.

Also, on normal, the final boss is quite easy. In fact, I've found multiple ways of defeating him with my team when I play the final chapter repeatedly. It's because when you beat the game, you can still continue playing using the same save file, where the load your last save point. You can continue to do other things like training other units, or sidequests and DLC maps before actually returning to the final chapter.

So, because Pairing Up is a core mechanic in this game, I used it heavily in every battle. And in the final boss, with my best pairings, I could one shot the final boss in the same turn I face it. And I've made a list of people who can do so:

1. Our main character Chrom. He does this easily because he obtained the final boss' weakness weapon two chapters before the fight. In this playthrough, Chrom was paired up, and was able to deal two critical hits on the boss with that weapon, easily defeating the boss. 
2. Henry the Sorcerer paired with Ricken the Sage. This game has a forging system, so I forged a tome for Henry which I use for the boss. The initial tome itself has a 50% chance of dealing a critical hit. I made the damage output of the spell even higher. In the playthough where I one-shot the boss with Henry, I need the final boss to land non-lethal hit on me. Because Henry is faster, he could strike twice. And on the second strike, Henry activated his ability, Vengeance, which deals additional damage to the opponent based on how much damage he received. Coupled with a critical hit from the tome I forged, the boss is as good as dead.
3. Lon'Qu or Owain, or even Lon'Qu or Owain paired together. Both of them are Swordmasters, and I forged them a blade that has a high critical hit rate. They both have the ability Astra. When activated, their first strike is a five-hit combo to the opponent, where each hit deals half the actual damage, but you still have a chance to deal a critical hit for those five hits. Just activating Astra on the first strike and landing a critical on the second ensures the final boss' death. Imagine if I paired both of them together, then I'll see critical over critical on the boss.
4. Donnel, just Donnel. While I have not try this myself, I know that he's quite powerful, but not strong enough to take on the final boss alone. If I did pair him up with someone, and I use his best weapon, I'm pretty sure the boss will fall in one turn.

So, right now, I'll replay the game, this time I use a different gender avatar, which will give me more replay options. Then I'll try Hard mode, because Normal is quite easy.