1 or 2 focus mits.
Room to move around in.
Have the person with the focus mitts (defender) move around holding the mitt(s) at different angles while changing the distance between themselves and the person throwing the attacks (attacker!).
The attacker has to adjust to the new range and target angle as quickly as possible and still throw a good punch/kick/whatever the chosen technique for the exercise is (start with punches until you get used to the exercise).
At the most basic level the defender stands in place and only changes angles and distance by moving the mit.
Once both people are comfortable with this the defender can begin moving around the practice space, forcing the attacker to move around the area.
At a basic level the attacker waits for the defender to get set then adjusts position and attacks.
The next step is for the defender to give a signal when the attacker should attack. When the signal is given the attacker then moves and attacks. This works best if the defender has 2 focus mitts. By alternating mitts they can very quickly call for the next attack, forcing the attacker to adjust more quickly.
To increase the challenge have the attacker close their eyes after each attack and not open them until the defender gives the attack signal.
Give it a try and post here with your results. I haven't worked this drill in some time and I would really like to hear if anyone finds it helpful.