Ally with two of them, then tell them to attack each other. The winner will be weakened by the battle and you'll easily take him out. Use it as a bait to attract their army into a trap. When they come close to it, self-destruct. If they won't come, take out some harvesters.
Saboteurs are an excellent weapon, but they're even better when helped by a death hand.
Capture a harkonnen palace. When the death hand is ready, send it against the enemy defences. Before he's got time to regroup, attack his main buildings with a wave of saboteurs. This is the stealth raider rush. It's one of the best multiplayer strategies.