This requires you to go to a level that you have already completed and which has a tome chamber that you have opened. The first field you usually encounter one in is Field I3. You then have to complete the same criteria that you used to open it originally. In this instance
Field I3 - kill 146 swarmlings within the range of 7.1 - acquire Bolt spell
. Doing so will unlock the achievement once you have beaten the field.