Taming a dunklosteous is not too hard if you do this.
1. Get lazurous chowder or scuba gear
2. Have kibble or prime meat ready
3. Have at least 2 good crossbows. Kill some alpha raptors to get some.
4. Use a mount. I recommend a Sarcosichus or a Baryonyx. Make sure you can fire off of its back.
5. Find one. They typically spawn near the edge of the map on The Island.
6. Clear out any hostile creatures that are too close. They will interfere with your tame.
7. Shoot it full of tranqs. This may take a while. They are slow, so simply kite them while riding your mount.
8. Once they are knocked out, put kibble and/or prime meat in their inventory. They may need narcotics depending on the level. Make sure to protect them.
9. You now have a Dunklosteous! I suggest bringing a cryopod because they are very slow swimmers.
Hope this helps, good luck! :)