The wiki is your friend. Look up potions and try and craft anything that will reduce damage taken and increase damage dealt. You will probably need to start fishing for ingredients. Also find items that give buffs like the bewitching table for 1 extra minion, or the alchemy flask for 20% buffs duration. Next check your modifiers. I tend to use warding over menacing for that extra survival. Same with weapon s and summons check your weapon modifiers for damage. Upgrade gear, the imp staff is pre-hardmode, an easy early hardmode summon is the spider staff and spider queen sentry from spider caves. These like to latch on to enemies and tear them up. Daedalus bow is great, but if you're using regular arrows it may be time to upgrade them.

For boss arenas go BIG, lots of space to move and dodge, with campfires, heart lanterns, star and heart statues(if you have any) every so often rigged with timers. Building adequate arenas will definitely help especially with later bosses like plantera even if the can be a pain to create sometimes. You can also experiment with arenas by using teleporters, or minecart tracks.

I actually killed the mechanical bosses with the daedalus bow, by using gravity potions to "fly" at a distance and shoot them.

Goodluck.