Sensei is a Yellow Comet CO whose strength lies in overwhelming opponents with massive amounts of Footsoldiers and B-Copters.
His copters are super powerful, and his infantry are as well. His transports get +1 movement. With his powers, and footsoldiers being his only good ground units, you'll find yourself constantly spamming infantry and mechs into a wild flood.
He can beat any CO who cannot shield well against his swarms; on the flip side, he is countered by COs who can. Kanbei, Grit, and Hachi are good examples.
Power: Copter Command -- Sensei's B-Copters gain 25% increased offense and 10% increased defense, and all other units gain 10% increased offense and defense. 9 HP Infantry are created on every empty city Sensei owns.
Super: Airborne Assault -- Sensei's B-Copters gain 25% increased offense and 10% increased defense, and all other units gain 10% increased offense and defense. 9 HP Mechs are created on every empty city Sensei owns.
Zerg. Use your COP and SCOP to know what it feels like to enter the heart of the swarm. On a large map with a lot of cities, it will eventually become a chore to manage all your units.
Never forget to mix in some indirect units and Anti-Airs with your swirling mass of footsoldiers. Deploy B-Copters too, they're so much easier to deploy (Sensei tends to deploy cheaper units as he doesn't rely too much on tanks) and strong enough to beat weakened Anti-Airs, especially with powers.
You can join all your 9 HP infantry and mechs if you want some extra cash for buying that big shiny vehicle you've been wanting. Sensei has poor command of big shiny vehicles though; use with caution.
How to Counter
The best way to beat Sensei is to keep his swarms away; indirect fire, strong meatshields, heavy tanks, and air units all help. The point is to either to destroy the swarm before it reaches you, or be able to defeat it once it does.
Sensei is heavily countered by Kanbei, Hachi, and Grit in particular. These COs possess hard-to-damage units and superior indirect firepower and will make life easier for you if picked.