IPA: /ˌkætəˈklɪzmɪkli/
KK: /kætəˈklɪzmɪkli/
In a way that causes great destruction or disaster.
The earthquake affected the city cataclysmically, leaving many buildings in ruins.
Cataclysmically → It is formed from "cataclysm" (from Greek "kataklysmos", meaning a flood or deluge) and the suffix "-ically" (meaning in a manner relating to). The word "cataclysmically" means in a manner relating to a catastrophic event or flood.
Think of a 'flood' ('kataklysmos') that causes great destruction, and remember that 'cataclysmically' describes something happening in a catastrophic way.
No commonly confused words.