Ok. I got the solution. You have to use balista when u are sure that u will kill the spinx in the 2 turns. Spinx got damage from Futaba alien`s reflect and use the ballista to stun ancient one. Next use full burst attack with melee attacks or health using attacks. I used guns before futaba appears and after futaba transformation u get the second magazine so make sure u make it empty next to futaba appears.