I have no idea how to do it in 20 turns but I made it in like 170. I played quite early (turn 1-4) the sanction card on Durocortorum to prevent him to get more territories. 1st turn: Let your ally get Haemimontus in the first round to save you Thracia. Otherwise restart. Play the reinforcement and the prority card and put your troops on Rhegium. Move your troops from Cannae to it and attack Thracia. Attack Syracuse with both units to get rid of him for the moment.
In the next turns try to get Narona and Savaria and other territories also from your ally. Don´t take Rhodope cause of the -2. Leave your ally there and at Byzantium.
Rush to the north but avoid to get Aventicum, Colonia Agrippinensis, Narbo and Aquitania.
Have always something like 3-4 units on the other territories around Aventicum and stack up to 30 at Massilia. Go on to the south but avoid Salmantica and Carthago Nova.
When you have 60+ troops at Moguntiacum you can go for Colonia Agrippinensis and Leodium. Lure out the enemy (be sure to let it you how moves first in the next turn) and move back to Colonia Agrippinensis. In the same turn take Aventicum and in the next turn Durocortorum. Use the +10 bonus to get rid of this enemy.
Stack up your army up to like 300 and kill the troops at Salmantica and Carthago Nova by moving your big Stack between Saguntum and Acra Leuce.
With the 300+ get Salmantica and dance again between to territories to kill the enemy there. Get Qssonoba, Carthago Nova and Hispalis but avoid Gadaes.
Stack up to 800 or 1000 and move on to Cartenna.
Stack up again to 1600 and finish the game.
How to win Level: Tricky Europe V2: 8/1/2019 08:35:15
I recently goldstarred it by sanctioning Gadeas turn 3 and then diploing Gadeas and Qssonoba the next turn. The logic is that this will slow down Qssonoba from getting the sanction card so Gadeas will become unsanctioned before Qssonoba gets it and Gadeas will put troops on Q’s border and Q will use it on Gadeas instead of you as he default if there are no troops on his border. This means you have to take the 10 bonus without sanctioning Duro. It is not too bad with abusing AI mechanics.
How to win Level: Tricky Europe V2: 8/11/2019 21:05:58