Think of it this way. To clear phase 2 takes 15 + 30 + 50 + 80 = 175 stam.
So as long as your max stam is over 175, then it's worth it because if you really wanted you can use the fg to refill stam.
Also, you don't need to 3-star every stage, only the last one. And I think there's no RNG in it, as long as you don't have a super fast squad with 4 super slow squads, it should be ok... i think.