require tips when assaulting in a city

[Imp]

and the main problem is that when i succeed to dig out an enemy unit, it flees in another hex i cannot see, and it rally the next turn, so that i have to restart everything from the beginning the next turn...

To avoid this make a decisive push in one or 2 points then fan out if possible. These forward forces isolate city blocks stopping route/reinforcement, sort of surround on 3 sides or at least 2. Place your arty so it fires just behind the front line to minimise hitting you & be in position for where they rout to
hit hard. Patience city busting is slow work esp in the snow, let your arty work using smoke to block fire lanes as well. Far better to go block by block isolating as stated then sweep round than try a whole front approach generaly. If you can get a MMG or 2 round the back with LOS down the road to the block you are attacking seals it & persuades runners to keep going.
That being said city fighting always causes heavy casulties which is why armies normaly try & avoid it if possible.