You've got a few good ones there to choose from.
Demytha and Maneater are both good, and could be useful for Dreadhorn - but they will need high resistance for the hardest levels.
Also, Deacon and Shamael stand out - both very useful.
As for Iron twins, your best option is probably Geomancer alongside someone with a reliable speed decrease, and your 3 best healers/revive supports.
Your Arbiter needs more speed. Like 50-100 more, minimum. Not really relevant to anything you've asked, but speed is still top priority.
A quick tip for Dreadhorn - Most people use that Rat-king guy, but unfortunately you dont have one, just like me. However, I discovered a pretty useful mechanic which I havent seen mentioned anywhere else.
The frenzy set can really work to your advantage in that fight. Say for example you have Demytha in frenzy, and theres a few bombs on the board, and some are about to go off. As soon as the first one hits, your demytha should instantly gain enough turn meter to cut in and put out her block damage buffs to protect your team from the rest of the bombs as they go off. You could of course equip a different support, or even several of them, but this trick can certainly win the fight for you.
Sand Devil is pretty much end-game. I'd recommend finding a video guide for farming lvl5-7.