The fight will likely require numerous attempts to get right, and each unsuccessful attempt will burn through your ammo synths fairly quickly. You could use heavy ammo to take out your oracle quickly, but fail so many times that you run out just as you get it right and have to deal damage to Atheon. This is why I recommend using a special to take out your oracle. One that can reliably destroy an oracle without having to reload, and one that won't burn through ammo synths. The particular special I had in mind? Icebreaker. Magazine size of 6, more than enough to blow up an oracle. Ammo regen will cause you to have an almost full magazine when each timewarp comes (provided you don't use your icebreaker on supplicants or Atheon between timewarps). This way you get to save your heavy for the damage phase, and because of that you'll be worrying about ammo a hell of a lot less.