I've actually dumped all my eggs in one bucket for all 4 types to see how they affect gameplay. Here are my observations for each:
Melee: This is an excellent strategy if you know how to juggle and stay in the air indefinitely. Pumping all your stats in melee is excellent, since air combos can keep almost all enemies stunned indefinitely if you time it right. Probably the easiest playthrough for me.
Defense: I literally become a cross between a tank and a punching bag. The flinch and knockback kept me from doing much, not to mention it took forever to kill most stuff. I'd recommend upgrading this alongside another stat, or only after you max out another trait.
Magic: I found this was phenomenal early to mid-game, but once I started getting into end-game stuff, my mana couldn't keep up with the damage output I wanted. If you're part of a team that can provide cover, it's one of the best stats to focus on due to its versatility. If you're going solo, it becomes difficult late-game waiting for mana to recharge. If you're cornered and are forced to exhaust your mana in burst damage, you either run around like a chicken with its head cut off or use your melee for a whopping 1 damage.
Agility: Out of all my playthroughs, this is the one I enjoyed the most. Once you have full speed, you become a sprinting ninja with a semi-automatic bow/arrow. You can deal a solid amount of ranged damage, not to mention evading anything becomes incredibly easy. If enemies get too close you just sprint to the other side of the screen and resume your firing. Timing of the bow is key here- fire too fast and shots won't register; fire too slow and you're missing out on higher DPS. What's awesome is that once you max out this stat, you can actually juggle enemies by firing the bow just right.
Each character trait requires a different talent to master; I'd recommend giving each of them a shot and figure out which you have a knack for. Once that stat is maxed out, put everything else into defense or agility.