+Drones- I believe that the Advent's main planet side might will be drones. There will likely be a base model, from which the Advent produce them, very easily modified to new terrain and tactics. While some may be controled directly by Advent operators, I think that they'll have some sort of group hivemind. Instilled with this, the army will act according to the direct wishes of a single person, the basic order being carried out by a set of routines in the drone AI.
+Cyborgs- The Advent already have a history with body modification, so why not use it for warfare? By equiping a single being with advanced weaponry, they can effectively counter any effective resistence to their drone army. Honestly I believe this will be a prominent part of their army, with many low level psychics and non-sensitive Advent donating their bodies for implantation. During the procedure, the volunteers will have many of their humanity stripped away. All thats left may be little more than adaptive machine. Under control of the hivemind, the cyborg will recieve orders and proceed with them. However, when encountering an unknown variable in combat, the cyborg will still have the ability to creatively make solutions. Thus they would be the Advent's ultimate shocktroopers.
+Zealots- Nothings more frightening then having your best friend or lover stab you in the back. I believe that the Advent would capitalize on this and focus their pyschic energy on causing as much chaos in an attack as possible. They could infiltrate almost any branch of a planetry government. This would leave gaping holes in a worlds defense, as their greatest strategist and leaders are assassinated by trusted staff.
This ability could also manifect in a different way. By skillfully engineering a few sorrowful incidents between the citizens and their government, the Advent could control a world with little to no pyschic manipulation.
The Smoking Gun
It was just a typical day on Naeve and Sergeant Williams was out on patrol in the planet's capital city. "What a pleasant day" Williams thought as the sun's warm rays ran over his face.
Making his way through the busy intersections of the city, the lone TEC marine saw a child. The young boy was alone. Silently he observed the lad, leading him to the conclusion that the boy had ran away from home. "Hey little guy, are you lost?" he said as non-menacingly as possible, beckconing the child to come over closer so they could speak. Barely hearing himself over streets background noise, and Williams repeated himself to make sure the kid heard him. "We're your parents? Did someone hurt you?"
The kid gave Williams a sad gaze, and began to mechanically shuffle toward him. Slowely a perverse smile started to creep across the boy's face, conflicting his tearing eyes.
Williams returned the smile and moved to outstretch his armored hand, urging the child to do the same so they could get off the busy street. It was very loud. He saw it then, almost toy-like, a GAF-098 laser pistol in the boy's hand. "Son put down the weapon, it's not a toy, it's real." He almost yelled at the kid, stopping himself at the end, suprised by his tone. "Please, this isn't a game."
"I'm sorry." Whimpered the child as he raised the weapon, pointing it with his shaking hands at Williams.
The Sergeant couldn't believe what he was seeing. Finally his training kicked in and he raised his rifle to target the weapon.
"Will I go to Heaven mister?" the boy said.
"I-" stammered the soldier. He was too slow.
The child pulled the trigger, a blaze of red energy bounding from the barrel to harmlessly hit the concrete near Williams head. His aim was terrible. He was only just a kid.
Williams didn't miss however, pulling his trigger only a mere milli-second after the boy did. He aimed to disarm to the kid and did so admirably. His round obliterated the pistol and proceeded to tear the child's arm in twain.
Blood was everywhere, shooting forth from the boy's arm, a small drop hitting nearly every passerby in the busy street. Turning to see the source of this explosion of gore, the men and women of Naeve saw the smoking gun of Sergeant Williams. "MURDERER!" They chanted in unision.
Sergeant Williams agreed.
Naeve fell to the Advent nearly a month and a half later. It was the victim of nearly a hundred other of such cases scattered throughout TEC space. They together proved that you didn't need an armada to conquer a world. Only one little boy and a smoking gun.