I would like to start that while I'm the first to put this guide on here I didn't figure this out myself, there were a couple of articles on the web which I can't remember the name of that pointed out which enemy to farm. Although, by the time I wrote this the rewards have been reduced by a patch already.
To upgrade shards speak to a creature called Twillen in the left-most part of the Wellspring Glades. He will upgrade your shards for a price. Upgrading shards are quite expensive, especially as there is a multitude of other things to buy from various merchants in Wellspring Glades.
However, there is a way to grind out a high amount of light and that is to face a certain enemy in Baur's Reach. Just by the first Spirit Well, drop down the left until you reach a cauldron near the bottom. An enemy with a large hammer and shield will be there. I'll put a screenshot for the location at the bottom of the guide. You can use a number of tactics to fight him but once killed he drops around 300-400 spirit at a time. Within 10 minutes I had managed to accumulate 15K which was more than enough to upgrade all shards. The enemy respawn quite quickly and the furthest I had to walk to trigger a respawn, if it wasnt with a few seconds, was to walk over to the cauldron.
Abilities I used are below.
This was the last thing I did so I had maxed my health and spirit mana and already upgraded all my abilities. Helpful abilities are: Light Burst, Spike
and Spirit Smash
for attaching; Burrow
Launch is particularly helpful for when you need to return to the Spirit Well for healing and saving.
I also had the following shards equipped, which would definitely be more helpful upgraded but I did this with them not upgraded. Light Harvest, Turmoil, Bounty and Spirit Surge
Screenshots for the location of the enemy.