Here's a comprehensive guide to facing Messmer the Impaler in "Elden Ring: Shadow of the Erdtree" based on information gathered from various gameplay insights and strategies shared by players:
Location
•Where to Find: Messmer is located in the Shadow Keep Legacy Dungeon. You'll find him in the Dark Chamber, which is accessible from the highest point of the Specimen Storeroom.
Preparation
Build Recommendations:
•Elemental Damage: Avoid fire damage as Messmer has high resistance to it. Consider using holy damage in the second phase since he loses resistance to it after transforming.
•Spirit Ashes: Summoning a Mimic Tear can be beneficial as it distracts him, giving you breathing space. Other tanky summons like Luthel the Headless or Banished Knight Oleg can also work.
Gear and Spells:
Use incantations like Frozen Lightning Spear for consistent damage. The Aspect of the Crucible: Thorns can deal significant damage quickly.
First Phase Battle Strategy 
Attacks to Watch For:
•Spear Flurry: Roll right first, then into him to avoid the flurry, then roll into him again for the slam, and roll once more to dodge the ground spears.
•Orb Explosion: Dodge when he slams the orb, wait a second, then dodge or block the explosion.
Tips:
•Use the initial cutscene to summon allies if available.
•Dodge backwards when he jumps into the air signaling an area-of-effect attack with his spear.
Second Phase Battle Strategy
•Transformation: Messmer turns into a giant shadow snake, making his attacks more aggressive and widespread.
Attacks to Watch For:
•Snake Charge: Roll in the direction the snake curves.
•Orb Slam: Similar to the first phase but with increased damage and area effect. Roll away after the first hit and jump or roll for the second.
Key Tactics
•Use Summons: They can distract Messmer, allowing you time to heal or attack.
•Dodge Timing: His attacks often have a rhythm; learning this can significantly reduce the damage you take.
•Exploiting Openings: After his heavy combos or when he misses you with certain attacks like the grab or aerial strike, you have a brief window for attacks. Don’t get greedy; one or two hits then back off.
•Post-Battle Reward: Upon defeating Messmer, you receive the Remembrance of the Impaler, which you can exchange for his weapon or an incantation.
Additional Notes
•Summoning Hornsent: If you haven't killed him earlier in the game, you can summon him to aid in the fight by using the summon sign near the entrance of the boss arena.
Conclusion
Remember, the key to defeating Messmer lies in understanding his move set, utilizing your summons effectively for distraction, and timing your dodges and attacks with precision. Enjoy the battle, as it's designed to be as much a dance as it is a fight.
