Diablos is often seen as a wall for a lot of players new to the series. If you feel like you breezed through the game but are stuck at Diablos, slow down, sheathe your weapon and just run around watching his attacks. All of his attacks are extremely telegraphed and easy to avoid with an obvious wind up. For his burrow, simply run perpendicular to the direction he is coming at you from and he will miss if he is not enraged. If he is enraged and he does a burrow, you may still get hit by this because he comes up much faster and closer to you, so either superman dive to avoid the damage or use a screamer pod as soon as he burrows to give yourself a damage opportunity. If your weapon is sheathed and you just keep running or even just jogging in a circle around him, none of his attacks will hit you allowing you to easily learn his attacks and his tells. He hits very very hard, often hard enough to 1 shot you if you're fighting him without higher level armor so take it slow and get familiar with his moves. He is designed specifically to punish mistakes, and becomes one of the easiest monsters in the game once you learn him due to the telegraphed nature of his attacks. This advice is applicable to most monsters, so try this any time you feel like you've hit a wall and found a monster you can't beat.