To add it all up:

Zell is ridiculously strong IF you have the time and patience to wait an entire Armageddon Fist - an AF with 255 Luck, 255 Attack and ~.05 seconds interval can One-Hit (well... actually ~117-Hit... oh well One-Limit) Omega Weapon.

Irvine is very very strong, even without waiting minutes for one Armageddon Fist. Yet, he has a limit of 100 shots for each ammo.

Meltdown reduces the enemy's defense stat to 0, making each enemy equally vulnerable.

The chance for a critical hit is
(Luck + 1) / 256

Luck can only be leveled up by farming till you get 1 Dark Matter which can be converted into 1 Luck Plus.

The time you have in your limit break depends on your HP and on whether you're in Aura status or not.

Getting ~0.05 second-intervals in Armageddon Fist requires either extreme reflexes or simply a command-controller which lets you program commands. Using a command-controller, Armageddon Fist and Quick Shot are about equially hard, as you only have to constantly press one button at either limit break.

AP ammo is probably the best ammo for Irvine, as it is more than 5x faster than Pulse Ammo, and about 3x faster than Normal Ammo and can therefore deal some serious damage.

(Renzokuken + Lion Heart at 255 Strength can deals from 208.750 to 250.085 damage, but will only happen rarely. Deals about as much damage as one Quick Shot Limit Break without a limit, but happens, as already said, quite rarely.)