I've been able to kill even alphas with a crossbow and about 75 fire arrows. When the Deathworm comes out of the ground, shoot one fire arrow, then while its going underground, run past it (I changed the setting so some player stats were a bit higher) I had about 350% movement speed. Easy solo kill.
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.