The bosses are incorrect on this page. I never encountered Onyxia in two full runs, instead there is another dragon-like boss, whose name I forgot, who has the hero power for 1 mana to equip a weapon that gains +1 attack whenever your opponent (you) plays a card.
Just play as Boom and always pick armor gain or bomb shuffle. I breezed through whilst only having some difficulty with Nefarian.
The bosses are incorrect on this page. I never encountered Onyxia in two full runs, instead there is another dragon-like boss, whose name I forgot, who has the hero power for 1 mana to equip a weapon that gains +1 attack whenever your opponent (you) plays a card.