More fun red tank, Zephiel or Arden?

Out of the two, I prefer Zephiel. I don't have either built, but I've used both in GC and Zephiel is just a more fun unit to use.

Not to mention he's easier to start merging if you do all of his GHB stuff, in case you ever wanna do that.

Arden is better with less investment, but you are correct, mages absolutely shred him. He needs boots, armor march, or clever movement assists to avoid magic damage at all costs (including dragons). He's a great physical wall, and even most blue units can't take him down.

Zephiel needs a couple key ingredients to be good. First of all, a refined weapon, which can be an investment in itself. Distant counter makes him an absolute beast, but as an f2p player, if you get that skill, it's probably better to keep the og unit intact or put it on your favorite character. He can take a hit from green mages and red mages better than Arden especially with wary fighter, so he's a little more versatile and less dependent on positioning. He won't score a ton of kills, but he's a trooper.

It really depends on the player, personally im with Arden due to his hp and he gives the barrier blade some use against mages AND dragons.