Hunt Diablos with Lance is like stealing candy from an infant.
May be this is a good reason to try lance for fun.

Diablos hits like a truck. Depending on the weapon, if you are not used to him, I highly suggest go defend mode first. Use the paralyzing gunlance(the great Girros tree and he is weak to paralysis). Study his movements while blocking. Gunlance is slow, but very effective to keep your hp safe. Every time he burrows underground, run as far as you can and Superman dive or if you have enough screaming pods, shoot the hole and he will bounce right back up. Tip: the moment he goes underground before the tail goes under, stop attacking and start running. I am a DB main and I couldn't win him properly (since it is my first time playing MHW) so I switched to GL. Once I gotten used to his tactics, I switched back to my ice DB and I own him alot faster. Even quicker once I gotten Fire and Ice DB. You haven't run into Black Diablos yet. She hits even harder and her burrowing is more random than Diablos. Diablos burrows in a straight line but Black Diablos burrows zigzag. Good luck and hope to hear your updates for him.

Charge blade is a very tactical weapon to use. I can see you are quite used to it but there are a few moves to use the charge blade more effectively. You can try watching Arekzz videos on Youtube. He has a tutorial on CBs in which you can take some of the tips to use it better. Also I never played any multiplayer at all. I been solo-ing from start till finish(of course not including some multiplayer related monsters like Kulve and Behemoth). Since you are saying you managed to kill it a few times, I am guessing you are saying your winning ratio is alot worst than your losing, eh? Well, you better do be prepared for Black Diablos when you reach high rank battles. It will be a helluva ride. 

Every monster can be done easily in solo. Multiplayer just makes the monster stronger and more HP to chop off, in which I hope you realize it. Beginner high rank armor you can go with Zorah's. Once you fight with Odogaron High Rank( I highly recommend farm for Radobaan HR alpha armor which has bleed proof so you have to worry less about your hp being cut everytime you move), farm for his alpha armor. The armor's skill will be enough to bring you towards end game. If you have trouble with normal Diablos, Black Diablos will be a nightmare for you.