What i experienced with cavalry is to have some rivendell and gondor knights among the main rohan riders,
be mixed lancers and skirmishers.
Wargs are a pain towards standing archers, but usually they approach first, charging.
I ride out, take few down, and usually leave archers in "hold your fire" stance, so they deal with loose wargs,
as very few of these seem brought down by arrows, given their manouver capabilities.
I guess the high lvl of charge on wargs is to represent the loose warg's bite, still something to worry about
after its rider was killed.
I order "hold your fire" and archers to hold and cavalry (many bows among them) to follow me,
and i take the right flank of the mass. When enemy is at proper range, I order to open fire.
After a medium to close ranged arrow rain from archers in front, and cavalry from the flank,
I order cavarly to charge. This pretty much crushed everything down.
After that, i order everybody to hold fire and recall cavalry and order archers advance, and so on.
When wargs are too many i ride out, as i said, with cavarly, and kill as many as I can.
About trolls, i simply order hold fire, dismount and close the line of archers, let the trolls close in to medium range,
and order to fire, they just can't walk another step towards us.
Send cavalry against them when they're mixed with orcs/whatever is simply a mess.