If you're planning on doing the fights in mage armor, expect to feel like you're protected with tissue paper; they've got the lowest defenses across the board (except maybe summoner armors, which aren't even as commonplace as the other three).
As Night Of The Sun suggested, the Golden Shower (uses ichor, a Hardmode Crimson-only drop) would be useful for taking out the bosses. If you can lay your hands on a spider cave, farming enough fangs to make the queen spider staff will help even if you're not a summoner, as it spawns a spider-themed turret, basically.
Outside of that, you could try to get crystals from underground hallow for use in the crystal storm spell (the pellets are fast, like a megashark's shot, have a half decent base damage at 25, and cost 4 mana a shot, so you could spam it); you'll want some crystals anyway for the magic harp you can make after beating the twins (the notes from that pierce).
As a set of pre-hard suggestions, water bolt and demon scythe can try to be useful if you can't assemble anything else, especially if you can dodge well enough to not get hit by the boss you fight, as you try and have demon scythe's charge up spin hit them; water bolt's best used only if you have a solid block roof for wherever you're fighting to take advantage of its ability to bounce with cheap cost (but low damage).
Otherwise, spears and melee armor are useful to try and not die so much.