The Destroyer should be your first target but you may want to beef your character up a bit more. I would go for at LEAST full Mythril melee set. Although it wouldn't hurt to try in your Cobalt set since the Phasesaber might give you an edge. I would hold off on the other 2 until you have full Adamantite melee set. Don't forget Ironskin, Regen, Swiftness (optional) and a Bowl of Soup.
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What you want first and foremost for fighting all the hardmode bosses is a cross necklace most likely. It lets you melee the destroyer without too many problems. Just make a phasesaber and know how to step from one side to the other. And go nuts with the buffs. No reason not to.
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Seriously people, you really don't need all that crazy armor .... I personally say go against the grain like I did and go for and solo skeletron prime first. As long as you know how to dodge, he really shouldn't be much of a problem at all. You will need some good ranged or magic weapon to beat him though. I beat the destroyer last.
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i was able to beat the twins first(full cobalt) but i couldnt best the destroyer until adamantite or mythril and i didnt finally waste skeletron prime til i was full hallow
i suggest all warding accessories or all menacing or a mix of the two
a few accessories i'd recomend are cross neckalce, warrior/ranger/sorcerer emblem, philosiphers stone, spectre boots, cloud in a balloon, angel/demon wings, or star cloak
for the twins and prime i suggest these weapons starcannon flamethrower megashark - cursed/crystal bullets
for the destroyer i suggest flamethrower dao of pow really anything that can hit alot of places alot of times, several magic items and sunfury come to mind
as for potions, ironskin unless you want to be DESTROYED especially if you dont have REALLY high defense thorns if fighting the destroyer(lots of low damage adds up) but not with the cross necklace or else it'd be mostly useless gravitation so you can fly around while fighting to dodge(although this is actually counterproductive with the destroyer
now for boss tips skeletron prime moves around alot so he'll be the hardest
skeletron prime would be the easiest by virtue of least health(ignoring arms) but he moves around so much its hard to aim at his head, and unless you've lost all common sense you wont be fighting anyone but the destroyer with a melee weapon
kill spazmatism of the twins first because he deals the damage
DONT get both twins to second form simultaineously you'll regret it
DONT think that it'll be a cakewalk to beat the twins just because they are 13000 and 14000 and your at 400 health, spaz gets ALOT more powerful in second form, retinazor IS a cakewalk though
the destroyer will kill your NPCS unless your far from base
i wired a machine of 5 heart statues to a 1 sec timer, allowing me to constantly replenish health during boss fights(HP potions are all but useless during REAL fights, where a 45/60 second wait will get you killed)
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