Based on what? Obi Wan isn't really against force attacks as demonstrated when he fought Dooku, Malak at the very least equals Dooku... But is very likely above him in force power. He would destroy Obi Wan with the force and likely Cin as well. Then you have the lightsaber skills, Obi Wan is a soresu master Cin is the trainer of the accademy, yet neither one of them would stand a chance against Malak who was the Dark Lord of the Sith over thousands a lightsaber prodigy, and a warrior so fierce that he has been said to have turned the tide of entire battles, and even an entire war.
He from as far as we know only lost two fights both times against Revan, and beat the greatest Jedi lightsaber duellist in his era, Kavar. Malak would take these two probably easy.
With what? Cin Drallig's power is at best, at or below ROTS Anakin's, but his teamwork with Kenobi is anyone's guess. Obi-Wan, though clever, has a style that is all defense and won't be able to make any real assaults on Malak. Meanwhile, Malak is a sith lord who is shown destroying two jedi at a time with relative ease. He has tons of experience fighting other force users, and his tapping of the dark side makes him that much more dangerous.