I don't think that buildings can suffer, greatly or otherwise. It is true that you can use that with both animate and inanimate objects in a restrictive (aka definining) relative clause, but the verb suffer does not collocate, at least not intransitively, with an inanimate object qua subject. (This is a little confusing, but when I refer to an object in this post I am referring to a thing as opposed to either a person or the object of a verb/preposition.) I have seen transitive constructions with inanimate objects qua subject, such as The building suffered great damage from the bombing, however; but it sounds quite unnatural intransitively. Thus, I suggest rewording the sentence this way:
He saw people and animals that had suffered greatly, and several/many destroyed/damaged buildings.
The differences between several/many and destroyed/damaged would each be a matter of degree. If the buildings in question have been completely destroyed, to the point of there being nothing left structurally, you could use the adjectives levelled or razed.
One other alternative for people and animals that had suffered greatly would be wounded/injured people and animals. The choice between wounded and injured depends on whether, on the one hand, the harm or injury is caused by combat or some other kind of weapon-related violence, which collocates with wounded, or, on the other, an accident of some kind, which usually collocates with injured. (Injured can also also collocate with combat or other kinds of weapon-related injuries, so long as the context is clear.) If you want to strengthen wounded, change it to severely wounded.
I hope this helps.