Learn Finnish: Meidän kannattaa hyödyntää tämä äkkilähtö mahdollisimman pian. - We should take advantage of this last-minute deal as soon as possible.

The word mahdollisimman is the instructive form of the superlative mahdollisin, meaning "most possibly". Because this word already provides the superlative maximization, it is simply followed by the basic form of an adjective or adverb. You don't need to alter pian to mean "as soon as possible".

Why does the sentence start with the genitive Meidän instead of the nominative Me?

The verb kannattaa (to be worthwhile / to be advisable) acts as an impersonal necessity verb here. These verbs require their subject to be in the genitive case (meidän) and the verb always stays in the third-person singular. Literally, the structure means "It is worthwhile for us..."

If tämä äkkilähtö is the object of the sentence, why doesn't it take the -n ending (tämän äkkilähdön)?

This is a classic exception to the object rule that is good to remember. When the main sentence structure is an impersonal necessity phrase (like Meidän kannattaa or Minun täytyy), a complete object drops the typical -n ending and looks exactly like the basic nominative form.

What are the root words that make up äkkilähtö?