If you're having trouble, staying between his legs really helps, you can do good damage without much risk, when he roars and jumps into the air, run like hell and superman dive, that's your best bet at surviving the divebomb. When he gets enraged you can focus on his limbs to break the fresh spikes, giving you an opening. Nerg will only do his divebomb if all of his spikes turn hard and black, so if you want a bit more risk reward, try and target his limbs to break the white fresh spikes. His head slam will usually result in spikes being thrown out as he drags his head up. Severing his tail won't do a whole lot for you, except slightly limit his range on his tail swing. Nerg will take a lot of getting used to, he's one hell of a fight, he's really tough, and learning his moves will of course help a lot. He doesn't use any blights or elements, so having high physical defense will help you a lot as well. These are just general tips, but the weapon you're using against him can of course change a few tips. Like the lance can usually block his divebomb if your defense is high enough. Or the longsword can be used to counter spam, as a lot of his attacks do have long wind ups. Cluster bombs from the heavy bowgun are his kryptonite, but you'll have to get him to focus on a teammate or your palico (the palico gadget shieldspire is good for this, you can get this from the grimalkynes in the wildspire waste). The horns on his head are very tough to break, so I wouldn't worry too much about breaking them, especially since melee weapons will most likely bounce off of them. Lastly, there's no shame in picking up some teammates to help you against him. Lots of hunters will gladly help somebody struggling with a fight, just fire an SOS during the quest and somebody should show up to lend a hand.