Complex sentence: The task was given to No.9 and No.617 Squadrons who operating from a base in Russia from where they attacked "Tirpitz" with Tallboy bombs which damaged her so extensively that she was forced to head south to Tromsø fjord to be repaired.
Split into simpler sentences: The task was given to No.9 and No.617 Squadrons, who were operating from a base in Russia from where they attacked "Tirpitz" with Tallboy bombs. They damaged her so extensively that she was forced to head south to Tromsø fjord to be repaired.