There is only one realistic way. Have enough spenders in guild.
Or a theoretical F2P way: Order everyone to hoard crystals as much as possible. And everyone sticks to the order. As a new character is released, 1~3 person get assigned for maxing out the character. So your guild has at least 1~3 7* characters of any kind. And platoon missions luckily didn't require too much new/impossible characters. 45*/48*! :p Easier said than done!
Or another F2P way is to max out every attainable characters so you can do most of platoon mission & develop key characters to the max & mod them well so you clear all combat/special missions even without the help of maxed platoons