How does Mass Hysteria compare to this? I think it could often be better than this.
Not really compareable. Mass Hysteria often works like lightbomb and sometimes even better, but then there are times when the effect does not hit everything and you are screwed. Lightbomb however does deal with nearly every board, except with low health/high life-boards. This is more reliable, but not necessarily stronger. However not stronger does not mean weak: this is probably one of the best board clears that exist in the game and most importantly there is no real limit to it. While Flamestrike only deals 4 damage, this damage here scales always with attack and spell power.
I still think that this is better than mass hysteria, since it can deal with the 4 mana 7/7 LUL, 0 mana 5/5 MEGALUL and 0 mana 8/8 OMEGALUL.