Golden rathian G is not too painful if you have patience. I would suggest using a great sword(I always use king thunder sword). Golden rathian G has a LOT of health, so don't get frustrated that the quest is taking too long and make dumb mistakes. I mostly do my attack on the wings. As the rathian is fireballing, roll out of the way and attack the LEFT wing(which is your right). I find it easier then attacking the right wing because using a great sword, your overhand slash cuts at a angle that's perfect for the right wing. So when the rathian is fireballing, do tri-circle-tri, then roll away. Many times you can attack more but you'll more likely to get rushed so I wouldn't suggest it.
Silver Rathlos G is a different story from the high lvl quests, its more aggressive and goes into rage mode about 12 times each quest. The best way to damage it is to wait for it to fly up and shoot fireballs, then hide in the shadow, when he lands, uppercut the LEFT wing. Always check if the rathlos is shooting backward fireballs so you don't get hit.
Golden rathian G quest takes about 20-30 minutes. Silver rathlos G quest takes about 30-50 minutes.
Don't get greedy and attack a lot, and don't forget to bring as many flash bombs as you need when your fighting the silver rathlos.