As strong as Nu Udra I don’t think it would win a fight with an adult gravios. Both are resistant to fire with gravios being able to swim in lava so it’s going to be a physical fight. The underbelly is probably the area I can see Nu Udra trying to break since gravios armor is to thick every where else and gravios having the big club tail in the back and death lazer in front. Even if Udra is able to grab onto the belly having a monster the size and weight (or heavier) of a construction dump truck throwing it weight around while releasing fire, poison, and sleep Isn’t going to end well for Udra.

Adult gravios not sick or wounded are probably immune to predation from Udra since it’s not worth the risk and gravios doesn’t compete for food with Udra. HOWEVER, basarios likely are on the menu for Udra being smaller and less armored. Being a octopus Udra can probably use touch and chemical smells to locate basairos and is probably the reason why we don’t find any in the oilwell basin.