Serious Sam 4

I found the best place to get this was at the very end of the "Gates of Hell" level. Where you encounter a boss fight with a Witchbride, the arena is fairly small and comes with a handy ammo box to refill your C4. Damage the Witchbride with either a Rocket Launcher or Assault Rifle until she's around 25% health, then attempt to throw and blow up C4 on her. I suggest doing this on Tourist (Easy) difficulty to ensure smooth sailing throughout the fight.

You will encounter more Witchbrides in future levels, which die a lot easier as they aren't classed as boss fights. But Gates of Hell is your first guaranteed encounter.

Super easy to do in “Hell on Earth” – kill the final boss (Witchbride) with C4, there’s a box of C4 in the arena to make it easier. You can weaken her with other weapons first.

Witch is flying monster Witch-Bride of Achriman. I.E. one can be found at the colloseum in Morituri tesalutant, where are other side quest with achievements. When you get to her prepare C4, and try to track the mist smoke which follow her trails and after she appears throw C4 with RT and when it will be close to her detonate it with RT. I did it with 2 C4 on Tourist difficulty. Hope this helps.