Swtich a lot from seated to standing,when standing, you use arms, pull the pedals on the way up by bending you knee (not just from the hips).

Ride a "long" and high grade hills from time to time. After that, most hills become easier.

I use those :
https://www.strava.com/segments/4331780 (2.2km at 13%)
https://www.strava.com/segments/4098000 (2.5km at 11.5%)

These are my tricks, they work for me, that doesn't mean they'll work for you.