i find myself alone with the medic
Your teammates should know you're going for the kritz, and from where. Standard play include someone distracting elsewhere, or a player standing behind your med to get flashed if someone bombs/kill you, or both
when to pop, under which circumstances
as marxist says one uber must yield either the medic frag, or 2 other frags to be profitable.
Obviously when you run kritz and enemy runs uber you should have an uber advantage that your medic should track.
a rough rule of thumb to count uber can be : uber takes 50 seconds to build, kritz takes 40. So try using the kritz advantage whenever possible, it does not matter if it's not on your demo. Just be certain that traps are cleared before you go in, and make sure to have a back up teammate in case the kritzed player dies.
Remember you don't have to get the med, if you can get 2 easy frags, get them, without those players the enemy uber will be easily forced/tanked/avoided.
When enemy has uber, your goal is to use kritz in such a way that you force them, escape, then get yourself a kritz advantage, OR get 2 frags and attempt a 6v4 fight/an uber force.
That means go kritz on the flank to get 2 frags if possible, or go for a very long range sticky without committing through a choke point.
Most medics are afraid of dropping. Depending on their level they might just pop when they see a glowing enemy weapon, so it may not be needed to take extreme risks.
On some chokes (like snakewater saw or gully choke) it may be easier to get a kritz force using a soldier.
If you have time, getting a heavy/pyro can be useful to help you and your med escape their ubercharge, although proper standard play should be enough.
Finally, kritz heavy is very strong, think about it when you're stuck with kritz attacking or defending last.
TLDR dont go for hero shots on medic but rather pop on whoever can get the easy frags and then kite the enemy