Tier 3 engine building gives +10% campaign movement speed and says "The Spirit of Grungni will no longer disengage during battle". But the army ability still says 120 seconds duration, and it still does disappear during battle.
This isn't for the quest battles, but this is only for the objectives that spawn a thematic enemy army on the map and tell you to kill it. These enemy armies don't move, they just wait for you to attack them. They cannot replenish or enter encamp stance.
One of these armies spawned but I didn't have any armies nearby, and I couldn't spare the gold to recruit new units just to kill it. But I had a Thane nearby, and every turn I used the assault units action to slightly damage all the enemy's units. It's usually a useless ability, but since the enemy army couldn't heal, I was able to whittle down its health over several turns. Eventually I brought a single lord over with some regiments of renown, and it was an easy win.
Definitely the most dishonorable cheese I've ever done though
Playing as Malakai, it's turn 30 and I have 4 full provinces, but I'm only making ~1,000 gold per turn with two armies, even less per turn if I need to recruit an emergency army since I'm surrounded by Chaos factions. Gyrocopters don't take as many casualties as infantry, which is one way to even the odds against numerically superior armies.
But they do very little damage against large entities. Every battle I spend half of my ammo on marauder horsemasters (the only enemy units that can damage mine), it takes multiple volleys from multiple gyrocopters to get a marauder horsemaster unit down to half health. Against trolls or some of the larger demons from quest battles, the damage output is pathetic. Twelve full units of gyrocopters firing uninterrupted on one Keeper of Secrets, and it took 4-5 volleys from the whole group to kill it.