Endermen can be really easy to kill with the right tools and skills.

Skills:
- Always attack the legs (If you don't look up at them, they don't teleport)
- Only take on one if you are in a very 'open' area
- If you are not ready to take on one of it's blows, look right in it's eyes so it freezes
- If it teleports in mid strike, follow the particles so you know the general direction it headed

Tools:
- Some armour of some kind is always best
- And iron sword or higher is required or your gonna waste a lot of time
- Bows (as you probably know) are useless, don't use projectiles

You can optionally also have splash potions which will help out a lot.

But there you go, there are my tips to killing endermen. Hope they work for you.
Happy killing,

HairyJake