The hardest thing in MK2 that I can recall is just Kintaro.
For me, the Mk that gave me most trouble was MK3, initially.
That for me, was the hardest MK game ever. I dont really remember Mk2 being that hard, I won easily (till Kintaro killed me), and Shao Kahn in that game is a joke!!! Extremely easy to kill.
Hmmm....hard choice, cause all the MK games before MK4 were a little toughy for me to beat.
MK1: it's hard for me to fight 2 people. MK2: Mileena and Kitana are annoying. Kintaro is a bastard unless I use Kitana on him. UMK3: Jade is such a evil bitch!