Can man of war jellyfish or box jellyfish kill a killer whale?

No; portuguese man o'war, or bluebottle jellyfish in not a jellyfish but a colony of many polyps.

Each polyp is a single animal in the polypoid stadium (not medusoid), specialized in particular functions, and all together are joint to make a unique "body": the colony. The floating part is a polyp full of gas, called siphonophore; the "eating" part are polyps called gastrozoids; the stingy tentacles are the dactylozoids; while the reproductive sistem is provided by gonozoids.