Try to tame a low level one first so you can farm blood packs with it, it'll shorten the farming time for a high level bat.
You can also tame a Daeodon first and use it's healing aura when extracting the first few packs, the pig can also help you raise the taming effectiveness for future high level tames, just let it wander on passive near the Desmodus you want to tame, keep some blood packs on it's inventory and let it get picked, it'll raise the effectiveness.
Let it pick you afterwards and feed on your blood packs to raise the taming meter, when it's about to tame, be sure to check if the effectiveness is close to 100%, if not just let it pick the Daeodon some more.
Steps to Tame a high level Desmodus with 100% Effektiveness (you need a low level Desmodus to start) :
Step 1: Tame a Stego/Andrewsarchus and saddle them (the saddle on the Andrew is essential!)
Step 2: Tame a Moschops of any level
Step 3: Farm 400-600 Bloodpacks
Step 4: After finding your target Desmodus take out your Stego/Andrewsarchus and your Moschops
Step 5: Put all the Bloodpacks except for the last 100 in the Moschops (the other 100 you keep in your own inventory)
Step 6: put the Moschops on "Following: Lowest" and let it follow your Andrew/Stego
Step 7: Aggro the Desmodus by approaching it and let it grab the moschops, the Desmodus will eat 10 Bloodpacks from the Moschops' inventory and then drop it, this will raise its Effectiveness by 1-3% (repeat this until the Desmodus is at 100% Effectivness. Do not unmount while doing this!)
Step 8: once the Desmodus is at 100% Effectiveness unmount and offer yourself up to the Desmodus. It will grab you and eat 10 Bloodpacks and then drop you.
Step 9: after it dropped you immediately mount your Andrew/Stego again. And let it grab the Moschops again so it's Effectiveness goes back up to 100%
Step 10: Repeat Step 8 & 9 until the Desmodus is tamed.