Well, Erdrick would most likely be based around the abilities common to DQ's iconic "Hero Class", a class he actually invented when he hit the scene in DQ3. Their most iconic attacks and spells tend to focus around lightning, but the Heroes are capable of a lot of different spells and skills as well
(especially if they pull from things added to the "Hero Class" after DQ3).
Erdrick was the one who started the convention of DQ Heroes being known for lightning based attacks
(well, not all of them, but the ones who share and build upon Erdrick's original abilities), so they would have to include his iconic "Zap" spells which work in tiers [Zap, Kazap, & Kazapple].
View attachment 181757 In Smash, I imagine it'd act sort of like Pikachu's "Thunder" move, except it'd fall in front of Erdrick and could be charged in order to strengthen the spell.
Moving on, Erdrick would also probably have access to Kaclang
(another spell he introduced to the series). It basically turns him into solid metal, making him immune to all attacks for a short period of time. However, he's also unable to act during it.
View attachment 181762 In Smash, I'd imagine this spell acting sort of like a more defensive version of Kirby's "Stone" attack. It probably wouldn't do much damage landing on people, but would instead be used to quickly shield yourself from harm
(though you'd obviously be able to cancel it anytime you wanted due to Smash lacking "turns"). To balance it, I'd probably make Erdrick able to be picked up like a Crate
(though he'd break free as soon as the spell wears off on its own), this making it so you'd have to use it wisely.
You also have other spells like "Sizzle"
(which would probably act like a wave of fire rolling across the stage), "Boom"
(which would cause an explosion), "Snooze"
(which would put enemies to sleep), and a plethora of other spells you could potentially include. There's a lot you can do with Erdrick, so the possibilities are quite substantial.
Of course, they'd have to include one of the series' most iconic attacks, "Gigaslash", in some way. Basically, whenever it appears in the games, it's an extremely powerful attack in which they slash their enemies with a "blade of holy electricity". It's pretty cool, and there's an even more powerful version of the attack called "Gigagash" as well.
This is probably what Erdrick's Final Smash would be like:
I think that paints a good enough picture for some of the things Erdrick/Luminary could do, so i'll stop here.