The heavy targaryen knight is the strongest cavalry troop in the game, but balanced because they are extremely hard to obtain.
The heavy targaryen knight are a heavy cavalry force. They are mostly used to charge the front line of enemy troops, as they have heavy armor and they are not that vulnerable to the attacks of enemy infantry. You can also (as with every other type of cavalry) use them to attack the flank of the enemy.
One of the most effective tactics to use with the targ Knights is to have your infantry hold group, creating a defensive wall. When the enemy are advancing towards your infantry, tell your cavalry to follow you and flank to the left or right until you are behind the advancing enemy. When they are within charging distance, Charge both your cavalry and your infantry at the same time. (timing can be very important as ideally they need to hit the enemy at the same time) This will greatly weaken the enemy as they will be directing their attention to two directions at once, effectively halving their combat effectiveness.
It is recommended not to use these tactics in a hilly battlefield since the horses won't be able to run at full speed and will be taken down more easily. Instead try to let the battle take place on a flat piece of ground or keep them together with your infantry men.