Ok, first you should have like this epic battle music.
Next, if you want it to be hard then in the troops actions section, put something like 'Fools, I won't go down so easily!' and then it regains all it's health. And make it in like turn no' 5-9.
Last, make sure the boss is fairly hard. If it is too easy than it may look big but is a wimp. Too hard then noone can win even if on lvl 99.
Tips:
At the begging of a battle, make it put a message like, 'Come to die have we?' or, 'My children shall feast on your flesh!'
Hope I helped!