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Thismight help. And I emphasise might because it seems way too complicated for your task but it works and is made for 1.2. Essentially, you need some way of avoiding damage from your bosses and some way to deal damage.
In the linked video, a heart statue farm is used which replenishes your health faster than the bosses can deplete it. Other methods include using a low damage mob (aka slimes from a statue) to take up invincibility frames, and with enough defence (85+), you could even use a spike form the dungeon to avoid relying on mobs. Another less reliable way is to build a hoik track that can take your player around the boss avoiding their attacks, although this works better for flying bosses (twins, duke fishron, etc.).
Without knowing what bosses you need to defeat, you'll probably be best off using a ranged/magic weapon of some sort. Otherwise, if you have access to the temple, you could use traps to help defeat the bosses quicker. OR if you have the horseman's blade from the pumpkin moon event, you can set up a arena like this which handles all kinds of mobs/events.