Complex sentence: They know that the worms can hear them but can't see them, so they decide to use grenades as distractions to help them reach a tall building, which they ascertain that the worms couldn't reach them.
Split into simpler sentences: They know that the worms can hear them but can't see them. So they decide to use grenades as distractions to help them reach a tall building, which they ascertain that the worms couldn't reach them.