Start by increasing your mana. Every time you increase your maximum mana pool you will regenerate faster. Upgrade the arrows to make it easier to kill the enemy without having to create troops. If you must, use simple ones which have less of an effect on your mana. After reaching 6,000 to 7,000 mana, upgrade all your barracks and hings. Load up full mana, then attack with wizards and paladins.
Kill the first monsters that the enemy sends with the arrows. When you get around 2,000 mana, start developing the abilities and send like one or two paladins to hold the enemies while you get mana. When you develop every ability, you will have to get 9,999 mana. Let it load all. When the mana is full, send plenty of paladins and mages.
The Lord of Hell can be defeated by any of the threee races. The amount of units doesn't matter as much as the type and the timing. All ranged attacks will make the Lord of Hell stronger, but it will not trigger the ground-pound. Use them as needed, but avoid over-producing them. All units that are able to attack at close range will trigger the ground-pound. It is recommended that you do not use any of them at all. The Angel Knights' blue flame attack makes the Lord of Hell stronger. However, his sword attack causes much more damage which compensates for the blue flame. As for the Red Dragon, you will most likely survive without it when using the Elves or the Orcs. You will certainly need it when using Mankind though. The Flame Devil will trigger the ground-pound.For Mankind, you will need the Arch Angel to push your units forward, but to finish off the Lord of Hell, only the Angel Knight and Red Dragon must remain to face him. This is because the Arch Angel triggers the ground-pound.