Boss difficulty

So, I'm at the point where I've beaten the lamb and ???

And I'm a little scared of how difficult Mega Satan, The beast and delerium are after i tried to do Hush

I was actually very good at dodging hush but, the damage this mf tanked, was absolutely ridiculous.

I had good output, glasses + poly plus sticky bomb, and my damage was like a pool noodle vs this idiot. Then the continuum attacks came and, I couldn't last long enough

If thats the speedrun boss, how the hell am i gonna do the non speedrun ones