In the earlier Joust ROM versions, there was a bug that allowed you to easily kill pterodactyls as long as you could stand it. This bug was present in the red and yellow ROM versions, but was eliminated in the green version. You only get 1000 points for each pterodactyl, but with a little practice, you can rack up really high scores using this cheat.
The bug can only be exploited on waves that have the center platform, after the lava troll is present (the fire burning on the lava shows he's there). First, kill all the enemies except for one Hunter (the grey buzzard-rider). Then, fly low over the lava until he gets caught by the lava troll. If he gets free, try again. As soon as he is caught, land on the center platform. Stay close to the middle of this platform. After a while, a pterodactyl will come after you; just point towards him while standing on the platform, and he will fly directly into your lance. In a while, you will start getting three pterodactyls on the screen at once. The only difficulty is in staying on the platform; if several come at you from the same direction, they can knock you off. If so, it is probably best to kill the Hunter and try again on the next appropriate wave.