Get the Meteor Staff, easy win against the destroyer.
Get the Crystal Storm & Cursed Flames/Golden Shower.
Crystal Storm will be your megashark until you reach Plantera. Cursed Flames will be a situational weapon to inflict debuffs or hurt multiple enemies at once - most bosses aren't immune to cursed flames.
Golden Shower will last you through the entire game, if your world is a crimson world of course. It allows you to deal +10 damage.
Sky Fracture is mmmmmmmeh. It's good, but consumes too much mana, and is single target. Poison Staff is also okayyyyy I guess.
Spirit Lamp is great! Get it as soon as you can.
You should start hardmode with the Flower of Fire & Flamelash. BUT! You can also use Magic Missile. It is now INFINITELY better because it can damage multiple enemies at once (AoE) and will deal 25-35 damage to most enemies which is pretty decent.
If you feel like farming, head to the Hallow & get a Crystal Serpent from fishing. You can also try the Flower of Frost & the Ice Staff but they can be neglected should you have the CS, CF and Spirit Lamp.
GET THE CRYSTAL VILE SHARD!!!!!!!!!!! IT IS SO SO SO SO GOOD during solar eclipses. You will thank me forever. Just kill a hallowed mimic until you get it. Magic dagger is worthless in my opinion.
Then, to get better magic weapons, kill the Twins.
They are REALLY REALLY hard as a mage especially in Master Mode (from my experience). You need to make a long platform (preferably with planter boxes), cheese them by putting the nurse at the end and healing every now and then, and using a lot of buff potions.
The best way to kill them is by debuffing them with Cursed Flames, running around and using Spirit Lamp, then using Crystal Storm.
I suggest you kill Retinazer first. DO NOT get them both to their second phase. Focus on one, then kill the other.
The Rainbow Rod will be your main weapon against Plantera because it homes in and does a lot of damage.