Young Merlin on Super NES: Walkthrough, part 5

To start, jump into the hole. You arrive on a mine cart. The track is not very long but quite complicated, so here is the map and the path to follow (you lazy bunch). At the beginning, watch out for the holes. The cart turns left. Take the 1st down, 1st left, watch out for the holes, the cart goes back up. 1st right, 1st up, 1st left. Straight ahead, the cart goes back down and then bounces. It goes back up and turns right. 2nd down, 1st left. Let yourself be guided.
SUMMARY (just the turns you need to take): r, l, 1D, 1L, u, d, u, 1R, 1U, 1L, 2D, 1L.

The cart stops at the end of the cave and Merlin picks up the stone. Take the gondolas going up to reach the room where you found the key.

In this room, place the stone at the beginning of the conveyor belt. It will be validated by the machine then purified by the metallic glutton to give a pretty purple diamond that will have to be thrown once more into the water of the rainbow waterfall. But for now, you must leave this place, and an old bearded man blocks your way and throws eyes at you.

To pass, you just need to kill 3 series of 3 eyes. To do this, place your reflection, move away from it on the side opposite the eyes and shoot at the eyes that stay stuck to it. As soon as your reflection disappears and if you haven't killed the 3 eyes, repeat this operation. After these 3 series, he will gesture for you to come and fight. So go and fight with your bare hands and show him who you are. You can now pass and go to the waterfall.