The player is rewarded with the Earth Sword for defeating this boss.
The Earth Guardian is a golem composed of numerous large rocks fused together. It has a large bulk, and lacks facial features other than two prominent mandibles.
The Earth Guardian is slow-moving, but strong, and will initially attack by slamming its fist into the ground resulting in a shock-wave that pushes the player back, and causes substantial damage. During this time, it's wise to strafe around the boss, coercing it to attack, and then moving back to a safe distance once it slams its fists into the ground. After it does this, it's best to take advantage of its short recovery time by pressing the assault.
After taking enough damage, the Earth Guardian retreats and attacks from range by firing boulders in the direction of the player, but will then return to its standard assault. Dodging is the best tactic here; however, after taking a substantial amount of damage the number of boulders it fires increases, and it will also begin to charge at the player. Luckily, the boulders are not difficult to dodge, and after it charges, there is enough time to get some easy hits on the guardian, due to the slow recovery time.
The Earth Guardian symbolizes the subconscious resolution of Jack Merrick. Their battle is a metaphorical one; symbolizing the struggle between the player's will and Merrick's, both attempting to assert their dominance over the other in order to change reality.