To learn getting these combos as high as 40, and ideally learning to get high variation bonusses while you are at it, just practise a lot in the challenges after you unlocked all the special abilities and weapons. The first challenge even has unarmed thugs so you don't even need to pay attention to knifes/guns/etc.
Just remember not to get overexcited when you are on a roll; one click too many or a mistimed ground takedown can cause the end of your combo. Always be ready to counter and if you get some tough opponents (ie. with weapons), the best way to take them out is by simply performing a special takedown on them.
I found that my 2nd playthrough (hard) was just a breeze after I scored the "100% Complete" on Normal. Hardly got hit at all and leveled up fast from fighting rather than solving puzzles :)