Before you start the first battle with Isaac, make sure the second Innocent Devil you have is leveled up to Water Golem and put in defense. Then, equip the Rapier and continue. The Black Demon will mostly attack your demon, but it will not get through its defense because it is in Water Golem form. When Isaac twirls his spear and slams it down, jump to avoid the shockwave. After his combo attack, attack him with the Rapier to get in a lot of hits. When Isaac yells ''Kill him'', the demon will sink into the ground and follow you. When Isaac laughs that indicates that the demon will attack where it is. Do not attack Isaac while he has the demon attacking you. Wait for it to attack your demon, then attack Isaac. Isaac will also use a tornado. Run away when he uses it, then repeat this to defeat him.