First and foremost you should only ever use Cavalry to attack any boss. Other troop types will almost always result in wounded troops. When sending cavalry to a boss, never mix tiers as this will also result in more wounded. Ideally once you have 100k t10 cavalry is the best time to start running bosses. Once you start your next goal is to move up to being able to do harder bosses with less losses. This can be achieved by increasing the stats of your cavalry.
These techs are your first stop to increasing your cavalries stats. Maxing them out will greatly reduce your wounded and allow you to take down bosses with less losses and less troops overall. At the same time you need to at least craft the King's Spear and then later a Dragon Spear.
These pieces of general gear greatly increase your cavalry attack even without stars and refines. Once you star them and then refine them they even further increase your cavalry attack making taking down bosses even easier.
There is a massive power jump between B11 and B12. While a b11 can be done with t11s, taking down a B12 with t11s is near impossible. As B12 are the lowest level boss to drop Dragon Scrolls if you don't already have a Dragon Spear Scroll you will struggle to kill B12 until you get one. Expect heavy losses with T12s at this point. If your server is still very new it would probably best to skip T12 cavs and go straight to T13 as you will be able to more manageably take down B12 and once you do get the Dragon Spear you can be ready to upgrade it to the which will be your best in slot for running Bosses
Tip: Higher tier cavalry do more damage and thus you need less to kill a boss than previous tiers