Really? I found magic to be the easyCapra is rough as a caster (slow casts with comparatively low damage + getting hit even once while getting to the ledge probably means you're dead; magic shield works well, though), but most of the difficulty comes from him being an earlygame boss and not having many options yet. And yea, for gaping dragon you'll run out of casts if you go in with any summons.
Sorcery is fairly powerful in DS1, but it starts off slow. You'll eventually unlock some of the bigger and badder blue icons + some specialized mage gear, which combined make many enemies and bosses waaay easier than melee (contrast this with DS3 where mages kinda got fucked except for invis-cheesing through maps).
mode for Dark Souls, perhaps even more than pyro because you can literally only use magic if you choose; if you start with a caster and go directly to the Blacksmith in New Londo to buy his spells you have 60 casts soul missile spell and with upwards of INT of 20 you can kill pretty much any enemy in 3 4 hits for the first half of the game and the started catalyst can pretty much take you though Anor Londo and the Lordvessel
Dont know if because I am so familiar with Kapra that I wasted him with a mage by keeping my distance and peppering him with spells, that worked with The Gapping Dragon by having Solaire Tank him while I casted slower spells or switched to faster casts.... hell by that time you can pretty much have 90 soul arrow spells if you rescued the mage too