Ok, the first time you beat the game on normal and play through the
hard mode, you'll notice that enemies take a little more life when
they attack... This also holds true for your first encounter with Nelo
Angelo (the Dark Knight)... After you fight him for a while, he'll
ascend one level to the next platform... Arm your grenade gun and
shoot him through the niche in the wall where he's standing... You'll
notice that he block every shot, but your devil triggers also
recharge... Remember that you regain life while in devil trigger
mode...? Engage your devil trigger and regain some life... You can
just repeat the process until your life is completely full... This can
also be done when Nelo Angelo ascends to the third level, but you may
need to practice the positioning/aiming to hit him...