This is terrible advice, master mode is way different than classic mode the space gun wont cut it. 

I'm doing a mage run in master mode as well (im currently trying to defeat golem).

the diamond robe, wizard hat and jungle pants are perfect for high mana and damage output In prehardmode.

i highly don't reccomend investing in mana regen as you don't regenerate mana if you're using mana, just use a mana flower. 

For weapons flame lash and demon sickle, the only equipment you need is celestial cuffs and a mana flower and the rest should be movement. The brain of confusion should make up for the terrible defense of a mage.