Actually kind of a laughable encounter. I play Chargeblade, nowhere near any decent set yet (just finished story), with Chrome Fortress III, elementless and a focus on crit. But no matter how you build, I think Seething is a joke. It was my 1st fight with it and went very well. Simple advice: 1. stay near its legs / under its belly where neither tail nor neck bombs can hurt you. Occassionally it will drop on its belly but with a huge telegraph so you cannot miss it. This is especially important during enrage. 2. Take max blastblight resistance and decent fire resist (at least 20 to become immune to blight) and you are fine. I have encountered far less 'dangerous' burst damage than with, to take an example at random, Brachydios. 3. Deviljho is nice to have but not necessary. Watch them massacre each other, collect the drops, if Bazel leaves all you gotta do is drop a smoke bomb and call your tail rider. No need to deal with Deviljho in any way. If you are lucky he will actually follow you (by chance) and f*cking do it again. 4. Chargeblade, Lance or anything with a shield is helpful in the rare situations when you cannot stay underneath Bazel and it does tailswipe or charges at you. I was very scared of this fight but it's kinda easy.