Once you get to the altar the general store girl will unnote your essence for a fee of 2 gp each note, she is north west of the altar. Also make sure not to sell to her, but to use the noted essence on her.
Correct on all, however you forgot 3 things. 1. he is a low level pure. 2. he would have to run through monsters and 3. it isn't efficient until level 60 runecrafting.
do 14-44 Fires, 44-99 natures either using the unnoting ess with gen store lady method or getting 57 summ(I recommend) and graahking it.