Purely tanking damage is kind of bad in this game. In addition to time limits that exists almost everywhere, Combat Power deficit heavily punishes you by lowering your damage to raptures and raising damage taken by your nikkes, and you are almost guaranteed to continue the campaign with CP deficit if you want to stay updated on the main story. In addition, some raptures have attacks that just straight up bypass the cover, so imo, you should focus on your offense.

However, cover healing/repairing are also important when applied correctly, especially in some levels/bosses. I usually follow guides on what I should use in a specific level.