I've killed all of them... in expert mode. This is going to be long. I know this is late, I mean REALLY late, but if you happen to see this, then good for you, this might help you a little bit.
Make sure to use buffs for ALL of the bosses, it's pretty much necessary. Also, I played as the ranger class, so some of these guides might not work for you.
EoC: Make a huge arena with platforms, and run and dodge. That's all I can say for him.
EoW: Make platforms higher then he can reach. Slime mount is recommended. I beat him ranger, but I think it would be easier with weapons that pierce more.
Skeletron: The demon of expert mode. Make a big arena in the dungeon, and make sure to put blocks above your head, because skeletron doesn't fire his skulls if there are blocks between him and you, and when he starts spinning, run as fast as you can, because if he catches you, you're dead.
WoF: Make a huge bridge, stay away from the hungries, and use your weapons. I played as ranger, so I had necro armor. He is possible with lower defence armor like necro, trust me.
HARDMODE!
Destroyer: Super easy peasy, just use daedalus stormbow with holy arrows. It's incredible if your a ranger, he dies super fast, and you can get your next weapon
. I don't really have a guide, just use your EoC arena.
The Twins: Make a skybridge, kill spasmatism first, and let retinizer be last. Megashark and crystal bullets are probably the bet weapon to use, as far as I know.
Skeletron Prime: Take out the limbs, target his head next. Use the skybridge that you used for the twins. Just run away and your fine.
Plantera: Plantera is probably the second most annoying boss (second to the moon lord). She deals more damage now, and some of the old methods don't work anymore, so this is what I did. I used a super big underground jungle arena with platforms for the first stage, and used a long hallway for the second form. DO NOT USE THE PLATFORM ARENA FOR HER SECOND FORM, use swiftness potion and hermes boots/fuzzy carrot/blessed apple and run as fast as you can, while attacking her. Also, a flamethrower works really well for plantera's second form, because of it's piercing and burning.
Golem: Pretty easy, just use buffs and you are fine.
Moons: Didn't do them
.
Martian Madness: Stand in a box, and use weapons that can go through walls to kill the martian saucers.
Duke Fishron: Not to hard, just use the cosmic car key (if you have it) to go in big circles for the first and the second form, then when he teleports around on his final form, it sounds crazy, but just go up and down in the cosmic car while firing at him with your weapons. It actually works.
Lunatic Cultist: Just fly and shoot, pretty easy. Also if you have the sniper rifle, it's best to use it when the lunatic cultist creates clones, because chlorophyte bullets will home on to the real lunatic cultist.
Moon Lord: Pretty much impossible on expert. It's terrible, it's so hard. Just build a long minecart track and shoot, I don't really have anything else to say.
There it is. A guide for every boss in the game on expert. If you didn't understand anything of what I just said, then please tell me. Expert mode is pretty awesome, and you get some awesome stuff from it (aka demon heart). It's really fun, and challenging, all at the same time.