Pick a level with Kate available, I chose New Orleans.
The disguise in this level is on a rooftop part of the brothel, and the nice lady wearing it wanders back and forth amongst sleeping guards. Knock her out (or kill her, you monster) and take her disguise. You must equip the disguise by selecting it with
then pressing
.
Now you're all gussied up, choose a guard to torment. I chose a guard across the road from the southern tip of the market (from the starting perspective of the level); he is talking to a couple of civilians. In my experience Kate's attraction skill will not work on Long Coat enemies or the poncho wearing guys, they're much too good at their jobs to slack off.
Next, making sure the disguise is on, stand next to the guard and press
twice (once to distract, twice to lure) to make him follow you, drooling. Try to start moving as soon as possible to make the most of each attraction spell, and walk in a straight line away from the guard's start point (around 25m). When it runs out, simply head back and entice him again!
Once in a rhythm, this only took 25 minutes. Poor guy had a long night and found out the true meaning of blue balls
Top tip: holding
and then holding the
button is fast forward in this version, and although you can't seem to use this and walk at the same time, it can speed up the guard returning to his post and the skill recharge (this was my last achievement to get and I had no idea there was a FFW option up to that point!).