The strength of 'Basic Magic' entirely depends on three key things.
One, the 'int' stat or mental prowess of the sorcerer.
Two, the 'proficiency' in basic magic of the sorcerer.
And last but the most important, 'Mana Control' of the sorcerer.
A sorcerer who excels in all of these three key things will obviously be able to cast basic magic so strong that even higher-grade individuals will have difficulty dealing with him.
Jareth has the passive skill 'Sharp mind' which boosts his mental prowess when casting magic, and as he has always been very untalented at fire magic, he has always used basic magic to do his bidding.
Thus his proficiency in basic magic could be said to be in a league of his own.
With the addition of mana Singularity, Jareth's basic magic has strengthened a hundredfold, which means his basic magic is nearly at the level of a semi-grade 1 sorcerer now.
