The reason I took a grass move rather than a fighting one on chesnaught was that i thought that I needed the grass coverage more than the fighting. However, I can full understand why you would want drain punch over seed bomb.
Mega Aerodactyl is a major threat, as you said, and I don't have any immediate counters to it aside from the one time Alakazam and sucker punch, so a possible suggestion from anyone would be handy, and I'll think about it and I'll try to find a way around it.
Thanks