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Well, we don't know how you started. But, in theory, on a map of this size, you can level up your main hero to incredible power. And considering you're a Necromancer, constantly replenishing your army and also having Raise Dead (a spell of only the second level, but it permanently revives your guys), you should have several thousand skeleton archers by the time Orange appears...
If there aren't enough of them, you can basically use a simple and long-term trick: place your hero in front of the opponent's last castle (Yellow, most likely, as he's the most resilient on this map), so that he blocks the exit, and constantly attack the secondary hero that will appear at the castle's "gates." In other words, you're draining his resources (he, like a machine, buys a new hero every day to replace the one you're killing :D). And while you're doing that, your main hero explores the map, collecting all the experience (there are plenty of islands with artifacts and monsters, and in the upper part of the map, there's a whole bunch of Titans guarding the cartographer). And you build up your army... well, and if you've already defeated everyone, just accumulate your army in the castles and money, send secondary heroes around the map, slightly leveling them up with "visits" to various locations, so that by the time Orange arrives, you have a good army in all your castles - if he doesn't stand up to you, he'll at least give the enemy a good shake.
But as it is... the map is simple, and I don't even know what could be difficult here... :confused: