Thanks to ox0 Shad0w 0xo for the private message. I tried for hours to get this and the "Eagle-eyed" achievement. Here's the quote and I hope it helps others:
So I was like you and tried forever the "Eagle-Eye" and "The Heat is On" achievements. This is what worked for me and hopefully helps you to. For the "Eagle-Eye" achievement normal bullets from human enemies do not seem to count at all. It is only the single projectile attacks (mainly from the Ancients) that have to be deflected back and hit the enemy. Personally got that achievement against the first boss doing a "perfect defend" against his sword wave projectile attack and deflecting it back causing him damage. Now for "The Heat is On" I could only make progress using the Rocket Charge attack (the one where you sprint and charge it up turning into a fire attack) against the Ancients. Shotgun Shrapnel didn't work for me. Anyway I hope I explained this well and you get your achievements! Good luck!