The best time to farm Delex items is when there is an Outbreak of Delex, when Delex will apear around 1.5x more than usual. This info can be found on the "Locale Info" section of the Expedition Tour. Always check the Locale Info to maximize opportunities like these.
Delex - How To Attack
Draw Out With Sonic Or Flash Bombs
Delex can usually be drawn out by dealing damage on them, but if you find you have a hard time landing hits, try using a Sonic Bomb or Flash Bomb to force the Delex out of the sand. Note that Flash Bombs won't work unless you throw them directly at Delex's head.
If you neither have a Sonic Bomb nor a Flash Bomb, you can also try attacking Delex while riding your Palamute so you can chase it as it moves. If even just one direct blow is given, Delex will stop moving to attack you.