Infernax

Here's a video of the location and walkthrough on how to do this.

1. Walk left from Arkos until you find a wall you need to break, and you will need to do 2 boss fights to be able to break this wall.
2. Gather all four skeletons as best you can.
3. Figure out your damage level. The skeletons were two hit kills for me, so I hit them each once.
4. Reflect the spear or throwing axes back at all 4 skeletons.

I managed to do this in Arkos Underpass on the screen with 3 or 4 skeletons and blocks you need the block break skill for. You also need to have upgraded your weapon enough to deflect projectiles back (can be bought from blacksmiths in towns). Carefully break the blocks without killing any of the skeletons and bait them all together in a group on the same screen. Wait for the one with the spear to throw a spear then deflect it back and it should go through all 3 skeletons and kill them.

One other way to do this is when you have holy light/hellfire, go to the bridge to the left of Arkos at nighttime, hit once all black jumping enemies (don't kill them), then go to the left side of the screen, use spell and when the skeleton throws the spear, use attack button.
The projectile will reflect the spear which gives more room for timing the attack.

You get the spell in last castle (Kadjanto).