To beat this one and get gold star you have to empower your partner (AI 2) from the beginning. Don't let him loose ground to AI 4 (close enemy) and have him attack AI 3 (top enemy). Completed in 21 turns using that method. You can also assist useless (AI 1) to break through the 1999 barrier around turn 10 to reach enemy but not sure this was necessary. Simply airlifting to Ai 2 would likely achieve the results. Good luck...

thank you I will try that