2. Manage Your Flask Usage
Flask of Crimson Tears (HP) is your primary source of healing in this fight, and you’ll need to use it effectively to Elden Ring Items stay alive during long, drawn-out phases.
The Flask of Cerulean Tears (FP) is for magic users, and you may need it to best place to buy elden ring items restore your spellcasting if you’re using powerful ranged sorceries or incantations. Make sure you’re conserving your FP for key moments, such as when you have an opening to deal damage or cast buffs.
General flask strategy:

If you’re running low on health but Mohg is still in the middle of a combo, avoid healing until the combo ends. Use the i-frames (invincibility frames) during dodges to safely heal once he’s not immediately threatening.
As you progress in the fight, don’t waste flasks on small amounts of health. Try to wait for larger openings to fully heal, rather than taking small sips between dodges. The goal is to make sure you have enough health when the next big attack lands.
3. Consider Flask of Wondrous Physick
The Flask of Wondrous Physick is a great tool for healing in this fight. This flask can be customized with two different crystal tears, and you can use it to give yourself temporary buffs or healing effects.
Recommended Crystal Tears for the fight:

Opaline Bubbletear: This tear provides damage negation, which can be especially useful when you’re healing. If you’re struggling with Mohg’s blood tentacles or AoE attacks, this tear can help mitigate damage while you focus on healing up.
Healing Tears: These can be a good option if you want to supplement your Crimson Tears, as they offer additional healing when you drink the flask.
Flame-Shrouding Cracktear: You can use this for a temporary boost in fire resistance if you’re planning to use fire-based attacks or spells. This will help you endure Mohg’s fiery Blood Boil attacks.
Bonus Tip: Don’t forget to equip and use your Wondrous Physick at the start of the fight to get the healing effects quickly, especially after you get caught in a combo or low health from Phase 2's AoE attacks.