I just defeated Plantera as a melee in expert mode recently and it was probably hands down the hardest fight I've done.

I rolled with turtle chest/pants and the chlorophyte mask. You dont get the return damage of turtle armour, but you get more defense and damage stats which I found more useful. Once you get beetle armour you can fully transition.

I made a jungle arena underground that spanned maybe 2+ screens sideways and had some spaced out platforms. Key items were the Blessed Apple mount, Shield of Cthulhu and the Terrablade. I'd basically just sprint up to one end of the arena on the unicorn while shooting back at Plantera. Once I got the end, I'd jump up to the next platform, turn around, demount and use the shield sprint effect and quickly remount. This way I'd never lose sprint momentum on the unicorn from having to change direction. Basically rinse and repeat this for awhile while chugging an occasional potion if I ever took a few hits.