Naturally, boss medals are harder to get in Hard Mode. Of all the bosses in normal mode, only Eligor truly troubled me enough to forgo his boss medal, but in Hard mode, Lv1, fresh especially, I have to leave many bosses un-medaled:
Goliath: Too many debris
Albus: His shots are a little too quick
Barlowe: His penta-dash is harder to dodge
Eligor: See above. Even with Death Ring, the battle drags on way too long for me to stay calm.
My suggestion is that if you REALLY want to collect the medals, play multiple playthoughs (via New Game+). Double Death Ring makes killing bosses often too easy....