The orb requirement on this mission especially on Hard and Very Hard is stupidly high for no reason (4000 on Hard and 5000 on Very Hard), because of that I had to find a way to get an S in orbs which involves doing the following: Secret mission 3 times, killing both small gigapedes and destroying the left of leviathan heart atleast once. Hopefully this helps anyone who has problems on this mission.
Nothing more to add.
Moving on...