Level 19 is right (or slightly over-leveled) for doing the Battle Tower - I assume you mean Laiken's tower, not Lovis' tower in Broken Valley. Orobas Fjords is what I consider "Act III". You're playing on Nightmare mode, and you're surprised that the enemies can be a nightmare to fight? Finish the Battle Tower, it'll help a lot.
I've noticed that Sosostra's chest does seem to give higher-quality creature parts.
The Ghost can cast a fireball spell, but it's really weak. Don't use the Ghost for the fireball spell. The Summon Undead spell has a better fireball, but probably not as good as the creature's.
I usually max Magic blast on my mage characters, but that's because they only have like 4 attack spells, and even then you should only really take three at most. Fireball is probably better that Magic Blast, it does high damage in an (eventually) large radius with the same cooldown as Magic Blast. Magic Blast does stun-enemies for a second or two, though, knocking them back as well.
Magic Missile is best used for a ranged character, but it is the best attack spell if you stay at range. It relies on a lot of smaller hits, which means you need to use it a lot, so you can either hit things in melee or with the missiles. The missiles shoot up and then forward, so it could have problems hitting things right up in your face - I've seen enemy missiles miss me when I was meleeing the caster.
I don't recommend mixing magic missiles and melee, you won't be getting use of half your build no matter which one you attack with.
