I think yudantaiteki's solution is optimal if you react with SC. But note that you can react with HT and have the same result in the end (ST and GS played, with a hand of SC, HT, Gold and whatever the two top cards were at the start of the puzzle), without the potential of screwing up what cards to put back. Of course, if you have no clue what the next cards are, you should check with SC reaction first. Maybe there are two more HT and you can react with all three of them...
With two ST in hand, first react with SC. Put back SC and HT. Due to GS, put back back Gold and one ST. Your hand is now ST,X,X. Play ST, draw ST and Gold. Play ST, draw SC and HT. As in round 1, this results in you having played your two non-reaction actions and having a 5 card hand remaining (consisting of HT, SC, Gold and top two cards).