Oops, I seemed to have accidentally binged on Terraria for 13 hours straight.
But hey, I beat my first Mechanical Boss, the Destroyer, by cheesing him through an elevated encased arena. The other two mecha bosses still annihilate me. I tried fighting Plantera but I stood no chance.
I also got, through sheer accident while farming for light souls, a frozen chest key! I immediately beset the dungeon, trying to remember where the thing was, and found it and now have an Ice Hydra staff! It basically allows me to put down an instant, high damage, stationary turret on a moment's notice whenever I need to!
I can cast him right through walls too, so whenever I'm like in the jungle, and I'm cornered into a room by like a dozen baddies with low health, I can just plant him straight in the throng of them and just watch and they get mopped up in no time. For 20 mana a pop, he lasts a long time too, like a whole minute, and monsters don't target him at all so he just sits there and fires until everything is dead. He's my new best friend, truly.
I'm trying to upgrade from Frost Armor to Chlorophyte, but I'm thinking I'm gonna have to contract the aid of my friends if I want to actually beat any of the bosses. Even with mad amounts of defense, I can still see these things tearing me apart.