OK, seems like raising ground 3 times and doing small "pillars" around the summoning altar really works. It really messed up bonemass and was easily able to jump out of the arena via those small gaps between raised ground. https://imgur.com/a/aO6AGiV

The fight lasted whole 5 minutes and it was easier than what I remembered. Fully upgraded banded shield was able to parry its attacks with ease. Those videos of other people beating bonemass to pulp would not had been needed but when in doubt ;-)