Thanks to the speedrunning scene, we now have several techniques to beat bosses within a few seconds. Parrying the Pursuer for time to use the crossbow, fooling the Dragonrider into falling off his platform... But the best, and most fun, is the stunlock method.
There are a couple bosses this works great against, but it's actually just a fun and addictive way to play the game. It's also very easy to pull off.
First, grab two clubs, or two maces. You'll be dual-wielding these. You'll also need to get your strength to 150% of the required strength for the weapon (18 and above is fine), so you can enter the power stance. This is done by holding Triangle or Y for a second. You'll notice a slight shift in stance.
In the power stance, L1 will do a swing with both clubs. L2 will do three hits in succession. Clubs and maces do lots of poise damage, and once your enemy's poise limit is reached, they'll be stunned for a few seconds — during which time you can start to do more poise damage.
Using the L2 should be able to poise break enemies over and over. Your stamina will be low, but just wait until you have a little bit left, then do it again. Don't be afraid to go into negative stamina. Bosses particularly susceptible to this are the Ruin Sentinels, and Mytha, but it can work great against others as well — even harder bosses like the Lost Sinner or Smelter Demon.
I prefer clubs because of the range, moveset, and A scaling with strength. It's easy to just keep upgrading Strength and get a lot of damage along with your stunlocking strat. Just remember that for the first second or so after an enemy is poise broken, they're immune to poise damage, so I usually just leave one breath before starting again.
There are speedrunners who use this technique only for specific parts of their run, and they're a lot better than I am (my best Any % time is just under 3 hours), but the following video illustrates the idea well enough: