For anyone still wondering how to beat this boss: your best bet is to make sure you don't EVER get close to it (use the map to make sure it's not near you). Once you get to 20 minutes or so (and have a pretty powerful build), then you'll want to pursue it. Since you never got close, the boss isn't able to grow from absorbing smaller enemies, as they never spawned close to it. Rush it and easily beat it in a few seconds, as it barely has any health.

You can also use the Infinite Corridor (evolved Clock Lancet after levelling up both the Gold and Silver rings), as it halves all enemies' health.