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Let's assume you have a random Heroes of Might and Magic map with an underground and no water (it's easier for me to think this way :rolleyes:). He plays as the Titans, and you try playing as Humans. I won't talk about the importance of quickly capturing Utopias and Pandora's Boxes here; it's obvious anyway. Just capture Griffins and, in doing so, accumulate Archangels. The Titans don't have this advantage, and it's not certain that he will have time to upgrade his Angels (if he's even collecting them), and even if he does, he won't have many of them. Since it's a random map and you've been playing for a long time, you have many castles and troops. Try combining your troops. I don't think you should have any problems.
As for the Necropolis... fight every battle (with skeletons), try to get the King Necromancer's amulet (in my opinion, it's not that difficult, although sometimes it's really hard to find). If you capture an Elven Castle, Hydras, and Black Dragons, you can turn 7 units into Skeletal Dragons. Also, collect Angels and upgrade them if possible.
In battle... use Prayer, and set up a counterattack. You can cast Berserk on him if it's beneficial, use Explosions, and Slow. Clone... well, these two factions should be able to defeat the Titans. They are the two strongest factions on random Heroes of Might and Magic maps.