Ok I tried beating the diablos from the guild quests and they are very..

You get the wings from HR2 basarios and up and diablos weekness is water

1.) diablos are weak against water. You can use sonic bombs against them when they dig. You can also get it's horns stuck in the ledges. It is also useful to chop it's tail off.

2.) you confuzzled me there. Those quests don't seem to be all in the same star. To get onto Hr 2 you only need to do 4* guild quests, you don't need to do 1*, 2* and 3* quests. The 4* quests you need to do to get the urgent quest '4 horns' is:
All quests except the mosswine arena and the capture the plesioth.

3.)Yeh, what ^ he said. From HR 2 and up basarios (5* and up guild quests)

OK WELL IT ISNT THAT HARD STAND ON THE LEDGE and it will try to ram you and get it's horns stuck atack from above use sonic bombs when they dig and pitfalls also when there horns are stuck to the wall jump off the ledge and attack the tail and when the tail is off get back on the ledge and when it rams you attack it's head eventually the horns will come off