Oh that's funny, I started a reply to this a couple days ago and forgot to ever finish it.
Attack damage is one of those stats deliberately enshrouded in ambiguity in order to make the game that much less number-obsessive. By not revealing the exact numbers to certain effects, understanding of the game requires a bit closer attention and involves a little more mystery. While a lot of exact numbers are available to the player, some things are put into uncertain terms in order to give the game a more natural feel to the machinery of it.
To my knowledge, melee damage descriptions such as extremely hard, massacre, and annihilate do correspond to a specific number range. A good way to gauge this is by noting how many HP you lose to each kind of hit yourself. I know A plain "hit" (or "slash," "crush," etc.) is about 7-9 damage while a massacre is around 20-29 damage. An annihilate is trickiest however, since there is nothing higher on the scale. A hit for 45 damage and a hit for 122 damage will both register as an "annihilate."
Spell damage can be hard to be exact on and takes experimentation. For some people, this way of requiring you to study the game makes it a little more adventurous. Sure it would be nice to know the exact amount of damage delivered by a spell whenever it hits, but if you play long enough to start to build an intuition for how effective your spells are.