The combo counter is the number in the top left corner of the screen. At every hit the combo counter will go up with 1. Easily preformed by holding
. This takes an amount of energy so try to kill some enemies when in need of energy.
Best done at Helm’s Deep level, when you need to defend the gate. This is where there’s a stair down and catapults flying all over the place. Head down the stairs and do
(energy must be full) over and over again and you should get over 30 hits in the matter of seconds.
You can also do this achievement on a ballista or the tower in Pelennor Fields. Just hit it untill you hit the 30.