|Turns out, Cordelia had the right idea all along with her base set, she was just the wrong person to go about doing it. This build gives Hardin an incredibly powerful player phase, while keeping his amazing mixed bulk. A highly unusual take on the character, and not one I'll guarantee will always be effective, but in the right hands this can do a lot.
First of all, the weapon of choice is the Shell Lance+, for the bonus stats it gives you when initiating. Not that Firesweep L+ would be a bad choice, but half the point of this is his bulk anyways, and he actually wants to take a hit in-between his barrages anyways. Picking the Res refine, along with a boon in Res, gives him a pretty solid base defense line of 36/39, which allows him to take on dragons as well as other melee units without a shred of fear.
After that, choice of special is really up to the player's choice. Iceberg is preferred because it's more consistent - Bonfire will be higher, but only slightly, and only when initiating combat. They're also chosen over Glacies or Ignis, as they are guaranteed to activate on Hardin's second attack, provided he doesn't one shot his opponent.
For the A slot, Sturdy Blow 2 is chosen because it combines beautifully with the refined Shell Lance, giving him incredible defenses of 42/41 when initiating combat. Death Blow 4 can also be chosen if you're happy with his basic defenses, for a great combined +8 attack when initiating combat. If strapped for either, though, his base Brazen Def/Res 3 will work perfectly fine - it'll dampen the effectiveness of his special on the first attack, but with some luck, you'll be put into range for it to be active on later turns after the first enemy's defeat, after which you can run with some of the most terrifying mixed defenses around.
B slot is simple - Bold Fighter 3 is the only real choice, as unless you really focus on it, his speed isn't good enough to double enemies, and even when you do he'll be unable to do so for the fastest possible enemies. Besides, he comes with it naturally, and this set's mainly about initiating combat anyways.
C slot, as always, is pretty open, and if you wanted to you could just give him a standard armor buff. However, with the Res refined Shell Lance, he actually has an impressive 39 base res, which makes him an excellent candidate for a ploy. Def Ploy 3 or Atk Ploy 3 are recommended, since the first will soften up the enemies he fights and the second will improve his ability to take the in-between hit even better, but you can pick another ploy if you'd rather him support another team member. This is also a good alternative if you have an ally with Armor March 3 to pair with him, so that he can fix his otherwise poor movement.
In a mixed squad, the Armored Boots seal is a good idea, to fix his ability to throw himself towards an opponent. Otherwise, just giving him a bunch of allies with Reposition to help is recommended. If you want to give him a different C, or can otherwise guarantee his increased movement, a ploy can be used here instead.