I know that feel being f2p myself...
but we must not be tempted by the shrine summoning...

though if you are in mood of doing premium summons ever...hold you SC
until 'Special summon' week comes..

During that week all the units you summon will be at LV 40...
kind of compensates if you get only golds and fodder silvers.. :P

bt in general dont waste SC...unless you feel super lucky