Ah, I understand. I'll try to describe how to get through that without giving any spoilers. The point of the game that you're currently at requires you to escape that battle. You aren't trying to win the battle or level up your spell, it will level up on its own after that particular battle. The idea is to use Shroud to allow your character to survive the battle long enough to escape.
I'll walk you through it if you don't know how to escape battles. Note that battle settings are changed for this mission, so if you had different settings you'll want to reset them after.
At the start of the battle, you'll want to use Shroud. Once Shroud is applied, you will then want to hold the button that allows you to escape combat while your character doesn't have an action ready. If you have everything set to the default settings, the escape button is the "Q" key on your keyboard. If you have an action ready, simply hold down that key and cancel your action.
Once you've escaped the battle your spell will level up.
On another note, if you happen to learn Cauterize during this mission, you'll want to follow essentially the same steps, replacing Shroud with Cauterize to deal with the poison.
I hope this helps, let me know if you have any more difficulties. Thank you for playing.