Glimpses are one of the two primary upgrade currencies in Mortal Shell. They are used to buy abilities for your Shells and are not lost upon death. Glimpses are generally rare drops from defeated enemies, but using the method detailed below you can farm 12 Glimpses and 1500 Tar in about 5-10 minutes.
Note: this method requires that you have completed at least one of the three temples in Fallgrim Outskirts to gain access to the NPC Thestus and the Fog Overworld.
Glimpse Farming Route
Starting from Fallgrim Tower, head upstairs and speak to Thestus. For the cost of one Glimpse, Thestus will bring the fog back to Fallgrim, covering the map in those demonic enemies, but also spawning two Grishnas.
Spend the Glimpse to activate the fog, then jump off the second floor of the tower and onto the ground below. Our destination is the river with the poisonous frogs, so cut through the enemy camp on the left and walk down into the water.
When you reach the river take a right follow it all the way down to the end where you will eventually find an enemy camp (where you previously found the Impervious Lute) which is now occupied by a Grishna. Kill the Grishna to claim 6 Glimpses and 750 Tar.
Retrace your steps back to the swamp, but rather than going back to Fallgrim Tower take the other path that leads towards the woods with the hanging bats. Fight your way through the enemies to reach the second Grishna waiting for you under the largest tree. Take him out to collect 6 more Glimpses and 750 more Tar.
From here you will need to pay Thestus one more Glimpse to get rid of the fog, or you can let an enemy kill you to reset the world and save yourself the Glimpse. Just make sure you die somewhere where you can easily retrieve your body!
To make your farm even better I recommend using Tiel, upgraded with the ability that lets you run without consuming stamina. This will let you traverse the map much more quickly, though more dangerous since Tiel has the lowest health of any Shell.
Note that there are enemies that drop from the sky at set points along this route. If you are running when you pass by their spawn point they will command grab you and deal a large amount of damage. Make sure to note these spots and walk as your approach them.
Finally, you can use a Bronze Bell item to increase the chances of Glimpse drops from the normal enemies along the route. This allows you to gain way more than 12 Glimpses per run.