What's the fastest way to level up?

I did a playthrough with a friend: he (orange knight) spent his whole experience in strength, while I (red knight) spent mine in magic. Most of time, magic seemed to deal more damages, but it was pretty hard to hit several enemies at once. Even if, most of the time, I killed more enemies than he did, he always gained more experience than I did (sometimes, almost twice more).

I already know that the Giraffey pet can help to level up faster (10% xp bonus), but above that, which choices can help to level up fast ? Are there any easier characters for that (with area magic skills, for instance) ? Is strength a better investment than magic ?

Regarding the red knight, I've read that the he was much appreciated for his "Lightning Hold" attack that can "splash damage", but it seemed really hard to perform: you have to be on the exact same Y axe than your targets to make it work...

You'll get XP by hitting enemies. One hit stands for ~1XP on insane ~2XP. If you deal less damage it still counts as 1 hit. So when you have low strength you can hit the enemies more times thus getting more XP. You wont get XP if the enemy is already dead (but you can still hit them). In arena you wont get XP either.

A formula to calculate XP needed for the next level:

XP for NextLevel = (CurrentLevel x (PrevLevel x 20 + 380)) / 2

The Industrialist and the Fencer have a multi-saw-blade attack that hits multiple times really fast. You can level them up really fast by just spamming magic.

(unlock with Blue Knight)

I already know that the Giraffey pet can help to level up faster (10% xp bonus),

This is true and will get you more XP by equipping him.
There are a few ways to level up fast.

Using the boomerang glitch on Industrial Castle. Throw the boomerang behind one of the standing poles when they come up. The boomerang hits many times per second. On insane mode you'll get like ~10.000XP.

Defeating infinite spawning trolls on Troll boss on insane mode. This is a good way if you know how to juggle and have low strength. XXY/SST* or XXYY/SSTT* across the screen will grant you ~20.000 a run. Try not to kill the boss. I've heard about people getting over 50k with this method.

(*) XBOX: X = X, Y = Y| PS3: S = Square, T = Triangle

I would recommend using Hawkster instead of Giraffey because Hawkster attacks enemies which will earn you extra XP. Hawkster also will get you fruit and sometimes burns the enemies. I have been using Hawkster and repeating the first levels with the barbarians. I have leveled up 15 times in 1 1/2 hours! (btw I'm using the pink knight with rainbow magic)