Get an ascendant shield and sword and back off when it makes a circle of sand then put out your shield and March forward attack two times then put up your shield and repeat until there is a dead Deathworm or get a Wyvern and park it on a cliff get an ascendent assault rifle and shoot it or make a pile of oil and light it up and make the Deathworm go through it countless times. Don't forget to pump some carry weight on you and your Wyvern and harvest away. P.S. When fighting the Deathworm for your own good hope you aren't squishy!
Since everyone else's tips are stupid. If you want to find a deathworm, you can use k mode and look underground. If a worm is nearby, you will see it's body. You can use a land mount like a thyla or a flyer like the wyvern. If flying, point the flyer downwards to make controlling easier. As for fighting it....use whatever dino you have. Rexes are great, wyverns too. Wolves are effective against the worms as well as the alpha to. If you plan to live out on the dunes in SE, be prepared to have random encounters time to time. These worms CAN destroy thatch-stone-adobe. My solution was to just use my rex army but wolves work great early game.