The Lord of the Rings War in the North

I found a good spot to go for this if you don't want to do a long playthrough as the LoreMaster. When going to the citadel in Fornost near the start of the game, there's a spot where you can return to Bree and have the plants respawn. There are likely many other spots you could do this process, but this particular spot only requires killing a single enemy by shooting an explosive pack on the platform they are standing on. Play through as normal until you hit this point and stop and continue the game with the LoreMaster.

It is just after the spot where you are trapped on the bridge. You'll get a prompt to go to the citadel and there will be an archer on a platform that you can destroy. Collapse the tower, harvest the two plants, then go to Bree and come back and the plants will have respawned. Rinse and repeat until you've crafted the 15 potions/food. Each potion/food crafted will require 2 plants.
This is a screenshot of the spot you are looking for:


--Here is where you will find the plants--
This one is immediately to the left of the where she is standing in the prior screenshot:


This one is to the left of the fast travel point:


Finally, while doing this, there are two chests that respawn if you want to farm some loot/money. Just go up this ramp created when you knocked down the platform (do not pull the lever until after you've got the achievement):

This is a pretty easy achievement. You have to be the Lore-Master, as she is the only character who can pick up wild flowers. You should be able to get this achievement by the time you reach the citadel in the first level, Fornost. Just pick up any flower you see available, then go to your second inventory menu, and pick two ingredients to mix. Do this fifteen times and your good to go. Make sure you are creating potions, not other healing materials.