Strategies for beating Pumpking as a Mage?

Solution 1:

The Spectre Hood armor set has pretty significant damage reduction (40%). I found using the Hallowed armor for boss fights (or wave events) more effective. I use a long-ish (about a screen length) area with a pit in the middle for traps. With this set-up, potions typically suffice for me.

I think your accessories look good, I usually use a similar setup. Depending on how much you're running, you may want to switch out the boots. I recommend something like Charm of Myths to reduce pot cooldown.

For weapons, I think the Heat Ray is good for the regular monsters, but you want something with high single-target damage for bosses. I've used the Leaf Blower fairly successfully against the Mourning Wood and Pumpking.

With this set-up, I can get 1-2 Pumpking kills a night. Not much, but it's a start. Things to improve would be reforging accessories for better modifiers, and making a better arena that kills the regular monsters for you, allowing you to focus on the bosses.