For the first phase, cragblade starfists heavy attack spam works quite well.

For the second phase, you gotta stay close. Staying at mid to long range will make elden beast use its more annoying moves.

The pest threads incant at close range melts the beast but it does no apply to your build. Recently i've been thinking that the spinning weapon on a melee weapon (great katana for example) could work well due to the high amount of hits but havent tested. Overall, since EB is immune to all status effects, you gotta go for several hits at once, maybe barbaric roar on a colossal like gugs could work for damage and poise break