"A larger version of the standard AT-AT, the AT-ACT walker features a cargo bed for the transportation of heavy building materials or combat munitions. This AT-ACT equipment allows the walker to summon stormtroopers in battle. Stormtroopers are summoned more frequently as the equipment is upgraded."
~In-game description of AT-ACT
An AT-ACT will often spell trouble for a unprepared rebel base as it will periodically spawn a free stormtrooper onto the field, allowing it to deploy a vanguard as it roams the battlefield. Much any like other unit-spawning vehicle, the AT-ACT is best deployed on the furthest reaches of the base, giving it time to grow a sizeable force as it moves towards its primary target.
However, it will have trouble still with negating Burst and Rocket Turrets, as it moves closer to the shield generators so having a small force of weaker units around it, is generally not a bad idea, to supplement its regiment of stormtroopers by its feet.
The AT-ACT has a slightly better effectiveness in Heroic Defense battles, as its foot soldiers can lend a hand and help get to enemy attackers quicker than itself. Likewise, let the AT-ACT move about in the midsection, of your base, as to give it the best chance at reaching the enemy.