So... I succeeded and got the bonus objective on the last turn, although I gave it up already around the 20th...
I took 5 panzers, 4x 3,7 mm AT, 4x 105 mm arty, 1x 203 mm, 3x 88mm AA, 1 recon, 3 He, 3 stuka core units
From the aux I took mostly panzers (the best ones), 2 infantry and one captured AA - biggest mistake was that I didn´t take artys instead of so many panzers
I started with the raid: 3 panzer IB, 1 T26 and the Verdeja ran from the central pass already in the first turn, destroying all artys and AA then I continued to the north part and destroyed the two big arty and quickly returned behind the river and moved the panzers down to the south.
In the meanwhile I constantly kept the air clean, one 88mm on the north, one at the middle (Arco airfield), and one around the southern-east airfield, all out of the range of the AI artys. I could reach all planes this way (with AA trait) and hunted down the weakened ones with the HEs.
The nationalists were on defensive stance during all 30 turns. The southern-west city did not need any support, they could easily hold back the waves alone. The biggest challenge was the southern and the east part, below Arco where I could not wipe all artys so quick like in the north. Around turn 15, I captured the middle southern city and launched another panzer raid with 3 Italian tanks and the Verdeja. It was extreme tough, I was constantly in encirclement and couldn´t move back behind the Ebro anymore. Hardly, but I wiped out all artys on the south but lost the 3 aux panzers, so I just moved up to the middle with the Verdeja. Alone it couldn´t eliminate the artys, but I always damaged one (every turn) and the AI repaired them always, so they didn´t shoot. The Verdeja was not attacked, because the AI permanently tried to break through.
By turn 26 I moved down most of my 105 mm artys and the Stukas and constantly attacked everything which tried to get across the river and kept the occupied cities under fire. Actually, the AI could not manage to get into the other side of the river (almost at all), I kept them in the entire length of Ebro on the other side. I timed all attacks to cause as much suppression as I can and attacked the cities with everything to kick the Republicans out. I finally succeeded on turn 30 and got the bonus objective
I could have done it better with more Italian artys, but it was a nice challenge this way.
I didn´t lose too much prestige with this tactic and kept the balance (on Colonel difficulty). On the last 10 turns my prestige was even increasing. I ended up the mission with over 18k in total.