In order to become a resistivity battle medic you will need this loudout.

Blutsauger
Vaccinator
Ubersaw

This loudout is needed for this type of battle medic and can’t be changed (with the exception of maybe switching to the crusaders crossbow)

The resistivity battle medic usually sticks around in the background picking off targets 1 by 1 (or multiple if they have the same damage source, for example 2 demos).
The strategy is simple, at roundstart heal one of your teammates until you have 2 ubercharges ready.
Then you can leave him to die and go into battle.
Flank what you can and look for single targets.
Right before you attack them switch to your vaccinator, pop the uber(bullet/explosive/fire depending on your target), then switch back to the ubersaw and kill them.
This will make it almost impossible for your target to kill you.
But you don’t have to just sit in the background!
Once you collected all 4 ubers you can just rush into the enemies blindly.
Try to pick targets from the same damage source.
If you first uber is almost drained quickly switch back to your vaccinator and pop another one,
you can continue fighting as long as enemies are coming since one ubersaw hit will give you another uber.
If you drop to low health quickly pull out the blutsauger and gain some health this way.

It will take some time to get used to this but once you have mastered this you will turn into a nightmare for the enemy team.

Here are some extra tips:

1: After every fight switch back to bullet resistance.
This will allow you to quickly react to weird sentry spots and heavys.
You will also be able to quickly switch to explosive resistance.
(which is the 2nd most common dmg source)
2: After a while people will try to start to melee you, this is where your melee skills come in place.
(I also recommend buying a key before every game to get the CritGods on your side)
3: Open, wide maps are best.

Gameplay video will come soon.