I played this on normal difficulty. This is my casual strategy, no sense of urgency in my moves as I explored the entire map taking all items in sight.
1. Travel EAST picking up every resource and buildings on route to the FIRST SYLVAN FORTRESS and take it. (Build up money structures or nothing for now so incase computer decides to take it back you don't want them to benefit from trainable troops)
2. Travel several turns to the EAST and get more resources and buildings without showing too much of the EASTERN map.
3. Go back WEST to get more resources and buildings on route to the WESTERN UNDERGROUND STAIRS.
4. Once underground, go SOUTH to EAST to the INFERNO FORTRESS and take it, taking more resources and buildings on route. (Also, use your remaining Sylvan armies more and slowly get them killed or leave them behind. You don't want mixed troops and get morale penalty)
5. Look at your day of the week. Make sure you build army buildings before end of the week otherwise make money and creature growth buildings first and creature building upgrades last. I took about 3-6 weeks doing nothing but build and train. (Hire those creatures that are supplied by the two buildings next to your Inferno Fortress on a weekly basis and upgrade them, they help. If you see the enemy hero on the move, try to intercept him)
6. Head back top side taking the SOUTH EAST STAIRS. (FYI: I went up because I saw the enemy hero headed towards my Sylvan Fortress up top. Anyway, I intercepted him and beat his army)
7. From the SOUTH EAST STAIRS, go NORTH towards the other SYLVAN FORTRESS and take it, taking resources and buildings on route. (You can have more freedom with what stuff to build here)
8. Go back UNDERGROUND through the NORTH EAST STAIRS and stop by your INFERNO FORTRESS for reinforcements.
9. Head for the WEST STAIRS from earlier that lead to the TOP SIDE and take SYLVAN FORTRESS in the WEST, NORTH of STAIRS. By this time you practically own the map. Just build up your miliarty and prepare for the last leg of the race.
10. Go back to your Inferno Fortress, reinforce, talk to the Black Dragon, bring at least 100 Hunters. Just train some and give to him. Now you can climb the NORTHERN STAIRS that will lead you to the FINAL FORTRESS and success.
NOTES:
1. The BLACK DRAGON is necessary to complete the mission. He's blocking the way to the FINAL NORTHERN SYLVAN FORTRESS. Stay away until the end.
2. It's possible to get the FIRST SYLVAN FORTRESS in the beginning before the INFERNO. It helps as it prevents enemy from amassing more troops.
3. To help you survive until you get Inferno Fortress, there are Sylvan troops scattered around that will join your army.
4. If you want to level up your hero quicker, sacrificed Lv 1 and 2 troops while in town for experience as everything else is enough to beat the map.
5. To stay alive, targeted range units first with Succubus Mistress, Pit fiend for clusters, Nightmares for heavy duty offense, Cerberus for medium duty offense, and Arch Devils for tanking and 1 hit KOs. Use gated troops and phantom forces spell to protect your troops or to make enemy waste retaliations. Play conservative, don't rush melee only enemies with your melee. Try to weaken them and pick them off first with your shooters and hopefully once in range, your melee can deliver the final blow.
6. You can train another hero to bring main hero reinforcements. Can be tedius but this allows you to keep going and going...
7. Don't be afraid to skip resources/buildings guarded by tough creatures, you'll have opportunities to come back to it later on.