You have to go to the Third Church of Marika in order to obtain the Flask of Wondrous Physick and a Crimson Crystal Tear that you will get with the Flask. Crimson Crystal tear fills back half of your HP. Flask of Wondrous Physick can be used to mix different Crystal Tears which will help you in combat. You can do a mixture of at a time two Crystal tears to use in your Flask of Wondrous Physick.
Erdtree Avatar is the boss that can be found at the big tree icon which is available in different regions of Elden Ring such as Limgrave, Liurnia. You can find them at the big tree icon and defeat them to obtain Crystal tears which you can then use to make a mixture in your Wondrous Physick flask.
Here are the different Crystal Tears you can find for your Flask of Wondrous Physick for mixing throughout the Elden Ring.
Strength-knot Crystal Tear
You can find the Strength-knot Crystal Tear near the Saint’s bridge in Limgrave. The Crystal Tear is present just to the right of the bridge in a bowl. This Crystal Tear is used to give you a temporary Strength boost when it’s used in the Flask of Wondrous Physick.
The bowl looks like as shown in the image
Dexterity-Knot Crystal Tear
You can find the Strength-knot Crystal Tear at the Folly on the Lake area there is a site of Grace there as well and it’s in the Liurnia of the Lakes region. The Crystal Tear is present in a bowl. This Crystal Tear is used to give you a temporary Dexterity boost when it’s used in the Flask of Wondrous Physick.
You can get this next Crystal tear called Opaline Bubbletear by defeating the Erdtree Avatar in the Minor Erdtree in Liurnia region. This Crystal tear when used negates the damage of the next hit an enemy will give you and will put a bubble around you.
Crimsonburst Crystal Tear
You can get the Crimsonburst Crystal tear by killing the Erdtree Avatar at the Weeping Peninsula Minor Erdtree. This Crystal tear when used will give you a bubble shield and also will restore your hp when you get hit by the enemy.
Magic-Shrouding Cracked Tear
You can get this next Crystal tear by defeating a Erdtree avatar at another Minor Erdtree in the Liurnia of the Lakes Region in the northeast.
You can use this Crystal tear and it will give a boost to your magic attacks while it’s mixed in your Flask of Wondrous Physick.
Greenspill Crystal Tear
You can get the Greenspill Crystal tear by defeating a Erdtree avator at a Minor Erdtree in the Limgrave region down south at the given location on the map.
You can use this Crystal tear and it will give a boost to your max stamina temporarily while it’s mixed in your Flask of Wondrous Physick.
Spiked Cracked Tear
You will also get the Spiked Cracked tear by defeating an Erdtree avator at a Minor Erdtree in the Limgrave region down south where you found the Greenspill Crystal Tear. You can use this Crystal Tear to give a temporary boost to your attacks making them charge attacks which increases your attack damages.