This is a video of me, Tasselfoot, beating the final boss of Epic War 2. While Guah Hajj and The End are fairly similar in strategy, Lord of Hell is very different.
To destroy the castle, do the same stuff as earlier levels. BUT. Make sure you don't overdo it with the number of guys you have on the screen at once. Too many, and Lord of Hell will ground pound you.
Lord of Hell does not move. He stays in place on the right side of the screen and ground pounds you and your castle (500 damage each time). He does this when you have too many guys on the screen... I think. We're not 100% sure what triggers it.
Also, ranged and magic attacks will be absorbed by Lord of Hell and he'll gain health instead of being hurt. So ONLY use Titan Units and Red Dragons for attacking... and 1 Angel Soldier at a time to keep the other units and cannons off your back. Do not use wizards/dryads. Do not use cannons.
I tried this 3 or 4 times as the Humans, including doing this exact strategy... yet Lord of Hell always ground pounded, regardless of how many guys I had on the screen. So I tried as the elves, and you'll notice after a few initial pounds, he doesn't pound at all the rest of the fight. Once he stops pounding, I start doing insane damage and take almost no damage. I actually get so many guys on the screen at once that I have to stop sending more guys for fear of having him start pounding again.
Summary: Use elves. Send Titan Unit, then Dragon, then Soldier (assuming the one you have already, if you have one, is almost dead). Then repeat. Your mana cycle should work out well so you can keep sending 1 of each (or 2 dragons instead of a soldier if you don't need to send a 2nd one). Soldiers heal the Lord, so only keep 1 attacking at a time.
Also, I did not use the F/G specials, as they do almost no damage. Not worth it. Who knows... maybe they trigger a ground pound as well.
Hope this all helps!
Video is on 4x speed, as it was 34 minutes real-time
Music: Ozzy Osbourne - Mama I'm Coming Home
Black Sabbath - Children Of The Grave