Slitherine

Have 9 infantry units left in January 45, Fallschirmjager, Pioniere and Bruckenpioniere. As city battles become more important after january 45 - which ones can withstand Russian artillery attacks, etc. the best?

Best wishes on your endeavors in the minefield that is the 45 campaign.

My suggestion is using Gerbisjagers. I found them the best to get and take punishment and their movement is pretty awesome.
I finished Berlin with 1 Pioniere 4 stars exp, 1 Grenadier Freidrich Pein 5 star, 1 +1 Movement Gerbisjager 5 star, 5 star Oleh Dir Gerbisjager.
In 45 I started using the handy cheap and effective, effective at absorbing damage, Volkssturm.

I suggest using those to help round out your army and fill in gaps on your map, and to help keep your soft cap down.
Use lots of artillery, if you need extra buy 10.5cm artys with trucks to fill in gaps, they gain experience quickly and are pretty good at repulsing infantry attacks.

I hope this helps.