for long term, I would almost say to focus your mag on dex; as a class not yet available can convert mag dex into all three attack stats. Until then; I'd say the best subclass for summoner is fighter, but braver is a decent second place too.
Light is usually best just because the Falzspawn are everywhere, but feel free to have some extra pets around in some of the other elements too.
Summoner's main selling points is that each pet can fill a different role of sorts; some pets punish overagressive enemies, others can dish out large amounts of damage to stationary targets, and others sill can fill a niche of keeping up with enemies that dash all over the place! I'd recommend getting a hold of each pet you can, and getting a feel for what their moves make them most suited for.