There are a few thoughts on world boss, but I can say with complete certainty, you need to play to your individual strengths against the world boss.

FIRE LORD - Cavs will be strongest against the fire lord while archers will be the weakest. However, if you have poor cav buffs and great archer buffs, you'll probably see better results with archers.

SCORPION KING - Ground troops will be strongest against the Scorpion while cavs will be the weakest. However, if you have poor ground buffs and great cav buffs, you'll see better results with cavs.

ELEPHANT KING - Archers will be strongest against the Elephant King while ground will be the weakest. 

WAR BEHEMOTH - Hit this boss with whatever is your strength, though it appears cavs are the strongest against this world boss.

The key is to play to your strengths as well. Most players have a focused troop type that has much better numbers and buffs. Attacking with these troops will generally yield the best results. But, you can hit it 5 times for free, and an additional 5 times for gem cost. You have plenty of time to test what works best for you, every week.  The crux of the issue is that you need to hit him 5 Times with even a few troops! It will get you gems and gold SO DO IT every week!!