You can pay for platinum summons with either cerberus coins or a platinum token. You will receive a free platinum as a reward for finishing chapter 3 of the main quest. Additional platinum summons cost 300 cerberus coins each (not including limited time offers), except for the first platinum summon of each event, which costs 60 coins.
When using platinum summmons, you are guaranteed to get a rare or above units. You can also obtain the special characters from both the current and previous special events. After you perform a platinum summon, the summoned unit will arrive as a gift.