you dont even need to cheese the best materials with a Reaver Shark to win against the bosses in expert mode. the only requirement is that you have a properly set up arena with campfire, heart lantern
and star in a bottle, with the most critical potions like regeneration, ironskin and endurance (swiftness isnt even needed for most of them because you dont have to run).
Eye of Cthulhu -> 250-300 hp minimum for the fight, and Platinum/Tungsten with some accesories if you want to get extra defense. you can sit perfectly still while killing it
Eater of Worlds -> Arkhalis shreds this guy so fast you can just stay still, if not, you can farm goblin scouts to make the item and summon the goblin army for getting the tinkerer and spiky balls, the balls kill this guy quickly because of the penetration damage to all segments.
Brain of Cthulhu -> this thing is vulnerable to knockback, so look for a weapon that deals constant DPS and can knock the brain and the seekers. swords like the Falcon Blade or Arkhalis do this. Diamond Staffs too, not sure on other weapons. contact with it give status ailments so the idea is to evade getting hit.
Queen Bee -> her attacks are very telegraphed, if she charges you jump, if she starts shooting stingers you dodge, if she starts releasing bees you kill them, the only thing you have to watch over is the poison, get a Bezoar for this fight. outside of that you can pretty much take your time to kill her, also, there's honey around, you get an advantage.
Skeletron -> this guy is the first one where i say you have to dodge, why? because of what the head does if it hits you, you really dont want to get locked with the head, take the arms down as soon as possible and start dodging. you need high defenses (probably start adding Warding to everything you carry), and regeneration sources to heal from the skull attacks. other wise its typical Skeletron, oh and dont let it get to the morning.
Wall of Flesh -> you'll need a weapon to take care of the hungry and deal high damage quickly because this thing gets fast when getting close to dying. the reason i look for crafting lava waders in every run is because being able to reach one of the edges of the map to summon this thing. you may want to make the hell bridge, but still, running half a map is the safest option, and even then, you still have to make sure to make enough DPS to kill this thing even if it locks you.
and that's expert Pre-Hardmode. first 3 bosses can be taken down as soon as Silver/Tungsten, Queen Bee and Skeletron require Crimson Armor (or that's what i recommend). WoF is more of a racing match rather than a duel, feel free to use any armor you want as long as you can shred this thing while being faster than sonic the hedgehog.