We found out that being opposite Willard is easier to take him down and get the artifacts. From the starting position, Willard is exactly on the opposite side and he will move to the either left or right. In any case, do not move at all.

Let's say that he moved to the left (anti-clockwise). Start shooting at him (you need about 12 Desert Eagle bullets) until he is stunned. Even if you see him close to you, do not move or you may fall into a lava pit. 95% he will fall between the hallway you are standing in front and the hallway on the left.

Once he is down, run to the right, pass the first hallway (not the second) and stand exactly opposite Willard (between two hallways). Do not try to get any of the artifacts yet, because you do not have time, as he is about to wake up.

Once he wakes up, you should be opposite each other. Start shooting at him again and if it is possible, side step to the right towards the next hallway (the one opposite the starting position).

Willard will fall again close to you, but you are also close to the hallway. Once he is down, roll and sprint into the hallway. Release just before you reach the artifact to avoid bypassing it. Pick it up, roll and sprint outside cutting to the right.

Willard should be on your left and if everything went well and you did not bump anywhere, he will be about to wake up whilst you are exiting the hallway. Stand opposite him (where the yellow X is) and start shooting (like you did before).

Repeat the above procedure until you get the four artifacts. When you get them all, the meteorite falls to the crater. Now Willard is vulnerable, so finish him off.