Well, the major damage is localized to that one spot it looks like...and you did mention that your burnt them with the light...I think.

But I can also see that all around the plant there is burnt leaf tips...that would indicate your using too much ferts.

Don't cut the dead stuff off. If you put them in 12/12 now, your plant will be flowering in a few weeks and all those large fan leaves will drop off on their own eventually anyway.