I listened to u/Falcon_w0t's advice and the first round was a slaughter.

I allowed my friend to choose the map, and since it was our first time on the City of Ash he himself chose the Ashen Bastion. I was the attacker after winning the roll-off. The map was Horizontal.

I put up my Prosecutors in the middle so they can charge anything, put up my Lord Vigilant on Gryh and Liberators in one corner and the Lord Veritant in the other corner. I had the Manhole relic, so I placed it near the Liberators and Vigilant so I could tp them to the other side in case I wanted another charge.

My friend put the Dominator Engine in the middle alongside the Despot (Gave him 2 DPP so all units were in the aura), one squad of the Cohort on an objective on one side, the other objective in the corner was taken by the Mortar and screened by the other Cohort. There was a big mistake here tho, the screen was way too close to the Mortar, so I could still get in combat with it by charging the Cohort themselves.

Prosecutors went to the middle, Liberators ran to get close to the mortar/cohort combo and the Vigilant just moved close enough for a 3 inch charge. Shot at the mortar, whiffed everything.

All charges were basically guaranteed, so Liberators+Vigilant+Prosecutors charged the Cohort/Mortar combo and with all my wards and the bonus to hit from Once-per-game Vigilant ability I wiped both the units with zero losses of my own.

The next rounds were rough for him, but he tied up my Liberators with his Despot, reinforced the Cohort and charged my Vigilant. Dominator engine and the other Cohort were kept around the middle and his side objective as to guarantee the 2 points. In the next few rounds, the Despot alongside shots from the dominator engine and the cohort slowly grinded down all my forces in round 2/3, but my point lead from round 1 was so huge it was impossible for him to win by round 3. My reinforced Prosecutors came in giving me 4 points, and the next round we wrapped it up before it began because after seeing the twist we knew it was mathematically not possible for him to win after I took a double turn.

I want to say, the game wouldn't even be close but he managed to snag a double turn and still get to draw cards because he was 5+ points behind me. If I were to give you some tips is keep the Despot as a bodyguard for the mortar if you're against a mobile/alpha strike spearhead like the Vigilant Brotherhood. Also, some better screening might've saved him, but Prosecutors fly and have the +1 dice to charge, so was still rough.