Killing a dragon is going to be one of the hard tasks in RLCraft. Not only do they burn you and shoot bombs at you, they are a tank. Before you fight one, you should level your defense and attack skill to 16 (Diamond level) and also level up your magic to use a Brewing Stand. If you already did the following you should have Diamond gear equipped. The next step is getting ingredients for a fire resistance potion, to get magma cream, you can kill lava monsters that spawn on lava pools, Salamanders are guaranteed to drop at least one or two magma cream. To get Nether wart, the best way to get it is in the Nether. In the Nether, nether warts are very abundant, but trying to retrieve one is nightmarishly hard as you will be swarmed by hostile mobs. Unless your portal isn't on an open space. The alternative and safest way to get Nether wart is by constructing a lit Nether portal, you don't have to enter it because it will occasionally spawn Belphegors that when killed, has a guaranteed drop of Nether warts. After you acquired the ingredients in making a fire resistance potion you can start brewing. Get three bottles and fill those up with water, brew them with nether wart, then magma cream and you're done. Optionally, you can drink grape wine to extend the length of the effect for longer battles. Additionally, you can add gunpowder so that you can also apply the effects to your flying mount. Recommended to add redstone for the 8 minute duration of the potion effects.
After you've done brewing your fire resistance potions. It's time to gear up. Recommended to enchant your diamond gear, but unenchanted ones works aswell. Get a stack or 2 of arrows, and an iron or diamond strengthened longbow. If everything is done you are officially prepared to slay your first dragon. Make sure to not die, keep warp scrolls or recall potions in your hotbar if the situation goes too dire. If you are fighting the dragon near something you don't want to be destroyed by flames, a diamond strengthened crossbow is a better option than the longbow because while they're not good in active or far ranged fights, they can safely out range the dragon's aggro, making it recommended if you fight those near your base or an important village.
When trying to melee combat dragons, It is very recommended to have Protection enchanted armor as they can hit bad when you keep your guard down. Food items like Paleo Salad, Blood Chili, Cooked Arisaur, and Chupacabra Meat are also needed, the leech effect the Blood chili and Cooked Chupacabra meat gives is a lifesaver as fighting a dragon will drain some hearts and hitting the dragon in melee combat will replenish lost hearts. Paleo salad and Cooked Arisaur meat amplifies the leech effect, healing more hearts during the fight. Potions like Fire Resistance are still recommended to use.
Ice dragons are more difficult to fight, as there are no potion effects that prevents you from getting damaged from their breaths, unlike fire dragons where their breaths are nulled because of the Fire Resistance effect. Fight one with the Speed effect which can be gained when eating cooked Jouste meat or Amber cake (A better version of Cooked Jouste Meat) and the with the Trueshot effect (An effect that increases projectile/arrow damage).