What's the best way to fight enderman in Nether?

The answer depends on your intentions, but there are two ways to defend against an Enderman.

1 - Avoiding: Do not look at their face. Even if an Enderman is standing directly in front of you, they will not become aggressive if you keep your gaze below their head. You can stare at their legs or torso without provoking them. A docile Enderman will never hurt you. If you don't think you can avoid looking at an Enderman, try lowering your cursor so even if one surprises you behind a corner you will not be staring at their face.

2 - Attacking: If you have to, or want to, kill an Enderman you can use your height advantage, or lack thereof in this case, to protect yourself. Endermen are three blocks tall according to the wiki. The player is just below 2 at 1.8 blocks tall. You can build a shelter or small floating platform three blocks high and stand underneath it in such a way that you can attack an enderman and they cannot hit back. Alternatively, the brute force method of killing an Enderman with ease, even on hard difficulty, would be to equip very strong Armour and use potion effects to your advantage.

Endermen can cause the following amount of damage without any protection:
♥♥♥♥♥.25 damage per swing on Hard

♥♥♥♥ and 1/2 on normal

♥♥.25 on easy

Making Endermen one of the most dangerous mobs to fight, and for this reason I would recommend Diamond armor or above if attacking with brute strength.

The absolute best weapon to fight an Enderman is a Netherite sword with Sharpness V, which on Bedrock edition can deal 15.25 damage without critical hits, meaning you can slay an Enderman in three hits. Unfortunately, I don't know the exact calculations that using a sharpness V Netherite sword with a Strength II potion effect would deal damage wise, but there is no harm in trying!

If you want to attack one in the nether, try to fight one near lava. If you manage to knock the enderman in, it will fall into lava. It will probably teleport back up, but since it is in flames, it'll weaken it a lot. I have tried this for myself, and it worked for me. You might have to be careful around the lava, as you don't want to fall in.

Another way is to put a pumpkin on your head after you carve it with shears, as the enderman will think you are one of them, and if this is not available, try not to look at them. If you want to fight them, this can be used as a sneak attack.

Also, if you can, try to put blocks over your head since enderman are very tall. If you do this, the enderman will not be able to hurt you, as it can't get to you, and thus, you can kill it without it hurting you. This can also help you in the overworld and the end, and can be used to make endermen farms.

The last way is to get the best sword you can (Which is sharpness V Netherite sword) and hit it with it, as this will easily kill the enderman. I hope this helps.