he is considered one of the best assassins in DC @AVP How many other characters can fight all three at once. Also he dint lose spectacularly. He lost but gave them a tough fight.
Deathstroke has been compared to Zealot in the new 52 in skill. Pre new 52 Deathstroke was skilled enough to take on Batman, Nightwing, and Robin (Tim Drake)
if you want proof on how he is a team anchor, look at the Justice League animated series. His death was the whole reason the Justice League became the Justice Lords in the alternate dimension.
Fighting wise the evidence is all over the comics. Its not worth providing examples. Batman has shown superior skill multiple times. Cap has shown superior physical attributes many times.
Batman has the skill to end it quick but if he doesnt Cap wins. Caps physical attributes make it an almost even playing field against Bats skills Its the endurance that gives Cap an advantage. If he draws out the fight, Bats gets tired long before Cap
he is smarter by about 9 times when it comes to tactics. Hence his increased brain power. The shield is good and all but only covers were he points it. The shield is not an all angle protection like Slade's armor.
Slade armor was able to withstand hits from Lobo. He charged at a helicopter that was shooting at him and the bullets just bounced off him. He also withstood a submarine being dropped on him. Physically he is equal to cap but is smarter and has a healing factor. All of it amped by his nth armor