Messmer the Impaler is weak to bleed, rot, and frostbite5. Using any of these or a combination of them can be useful in the fight. Weapon-wise, the Bloodhound Fang at +10 with the Bloodflame Blade can inflict massive bleed damage on him. Additionally, using the Bloodhound's Finesse skill can take significant chunks of his health. Alternating with Ezykes's Decay, which does rot damage, can also be effective.
The Mimic Tear Spirit Ashes in Elden Ring is a powerful summon that creates a duplicate of the player, including their equipment and abilities. This duplicate assists in battle by drawing enemy aggro, dealing damage, and providing a second source of healing. The Mimic Tear can be a game-changer in difficult fights, effectively doubling the player's combat potential.
In his first phase, Messmer the Impaler has several attack patterns. He begins with a thrusting attack and a follow-up fire burst. He also throws a fire spear from the air, performs a fire grab, uses flames that follow you, and unleashes devastating combos with area-of-effect fire attacks5. Additionally, he uses quick and frequent spear attacks and area-of-effect fire attacks when aggroed.