get your friend to run around as bait while you sneak up and knife the target also run around in circles 4 5 spins then turn around knife the target
If you are lucky you can stab one in the face, but that is rare. Another way is to shoot it 5-6 times in the leg with the Schofield (Bear) or 2-3 times (Cougar, be wary, it is hard to get leg shots) which should mean an easy knife kill
Cougars are quite easy to kill while you're on your horse. As soon as you spot a cougar just ride towards it as quickly as possible and slash with your knife. Most of the time you will get a hit and kill it, but if the hit doesn't kill it usually the cougar will try to run away from you as you hurt it, so you can just chase it down and, once again, just slash at it with your knife while still on horseback.
Same strategy can be used with the bears but you'll need to shoot them a few times to weaken them before starting to joust with them.