The thing about Taskmaster is that he is all about the moves and the acrobats, but lacks strategy. Sure he knows almost every move of every hero/villain in the Marvel world, but they are only good enough to allow him hold his own against them. Not too mention also lack of efficient equipment, for he only has a standard broadsword, a regular shield, a handgun, bow and trick arrows as well as a few other things. However he lacks the ability to think and create a strategy in the middle of combat.
Now Deathstroke on the other hand is a whole different story, even though he has increased strength, stamina, speed, and intellect, he never stays out of shape, for he is always training, and always studying on anything and everything, including his targets and possible opponents. So even though this will be a random encounter, no doubt that he will have studied up on him and become aware of his capabilities. Also he has far better gear and equipment, for one his armor and sword is made of Promethium and Nth metal, which in the DC Universe are two of the strongest metals. Also Deathstroke is a far better tactician and strategist, for even though Taskmaster can be on par with Deathstroke, Deathstroke can out think him in a matter of seconds. Plus Slade has gone up against even tougher opponents (even going up against several members of the Justice League all at once!).