What will cause a medic to pop uber is usually also what is best suited for outright killing the medic
Flanking is the best strategy here. Any class that can get behind the Medic's heal targets is in prime position to force an Uber pop. Which means the following classes are best at it:
- Soldier (rocket-jumping)
- Demoman (Sticky Jumping, more dangerous but effective)
- Spy (Preferably you will kill the medic, but forcing an uber pop is a good consolation prize)
- Pyro (Thanks to his Powerjack, Pyro can move much faster, and is already a class that should focus on flanking anyway. Plus, fire just has that wondeful 'fear' effect that makes a Medic want to pop his uber)
Ordered by preference. Scouts are especially good for this since they can slip behind the lines with a Bonk! and come back to harass the Medic, or just use their speed and jumping ability to slip by.
But a Soldier or a Demo will do in a pinch, since they can both fly above and launch an 'airstrike' on the Medic. Spies SHOULD be trying to kill the medic in one hit, but forcing a pop is good too (Snipers can do this too with a good body shot), and Pyros are just good at sewing chaos with flames anyway (though not the most highly-recommended for this task).
A Sentry Nest, a large enemy front, an Uber of your own, or any other major frontal threat will also 'make' a Medic pop their uber, but at that point it's more an effective use of that uber, rather than 'forcing' them to pop it.