I had pretty good luck with keeping giganteas about 6" under the water surface with the anemone directly under a 14k 150 watt MH, 8" from the water's surface, with a gentle surging current. Giganteas are often found in tide pools and other shallow water areas. It is pretty tough to over-light them. However, you don't want to shock them if they have been living in a low light holding tank, especially if they have lost a lot of their zooxanthellae, which most of them have.
I had some other problems that caused their demise but lighting wasn't one of them.