Get a 1h or a katana. Weapon weight decreases your effective athlethics fast, if you use a really light weapon you are super fast with 8 athlethics. Also, a 1h/2h of 1.2 weight slows you down more than a 1h of 1.2 weight, just like a 2h of 2 weight slows you down more than 1h/2h of 2 weight.
Go low on armour. Avoid the main zerg, get behind enemy lines and hunt those archers/xbowmen. Xbowmen mostly look around them while reloading though, so can't do a long approach on them.
If you want to assassinate key players you need to be in places where they do not yet expect you. So spawn fast, sheath weapon for supreme movement speed and then prepare an ambush.
I did shit like this all the time with my 15/24 ninja with steppe armor, feels so good when you get that sneaky kill, but apart from that very frustrating. Prepare to deal with horsies. I always liked to approach with my katana sheathed (looks badass) and then unsheath when really close to an unaware archer. See them turn around just before you chop is head in two.