Howdy. I am currently at start of Act 4, so I don't remember this puzzle in every detail - but it was hardest so far (took me over 1 hour to solve it).
First of all I encourage You to keep trying for the feeling of satisfaction that You solve puzzle Yourself - You can do it ;)
But if You want solution then I think is was like that:
Grey Rider, Grey Rider in second row. Then Wagenburg in first row (after jump it has 2 armor).
First War Wagon in same row as all units (first row). (Wagenburg has now 5 armor).
Secon War Wagog in oposite row. (Wagenburd still has 5 armor, Grey Riders are in second row now).
Move one Light Infantry unit to from second to first row (+3 armor), then more one Light Infantry from first row to second.
Repeat (+3 armor, grey riders are in second row - oposite to Wagenburg).
Play Reynard in first row (+3 armor).
Move one Light Infantry unit to from first to second row, then more one Light Infantry from second row to first (+3 armor).
Repeat (+3 armor).
This way I think Wagenburg should have 20 armor - kill troll with one command and lightning rod will not use its ability.
Hope I did not made any mistake - I have posted this solution using only my memory and piece of paper.