Sentence:  A building will <WSD>derank</WSD> one level at a time as its hitpoints are whittled away. So, while it may seem that the life bar is at its end, attackers need to take into account if they are merely <WSD>deranking</WSD> the building by one or whether they have taken it down to Rank1 and are about to destroy the building. 
Meaning of the word marked with <WSD> tags:  To lower or remove achievements and skill level.