In this video I mainly focused on base level cards. You can pretty easily play in silver without leveling up cards, so you don't need to purchase any summoners that you already can use for free. But yes, as you eventually climb higher you will need to level your summoners, so it is not a bad decision to do so. When a set sells out the first cards to spike are always the summoners, so I would try and get them leveled up as high as possible before Chaos sells out (still probably months away).