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The speed of the snipers is not important; what matters is placing Archers together with the snipers on the battlefield. Archers provide speed and, more importantly, resurrection for clones. Without Archers, Peplokryl will defeat the Titans with an explosion. You can try casting Antimagic on the Titans without shooting, while the computer spends all its mana on restoring troops/summoning elementals, and hope that the computer doesn't cast Haste. There are many options, in principle.
P.S. It would probably be better if you asked your questions about the walkthrough on
http://heroesportal.net/tavern/?id=402827&pagenum=13. I will try to answer immediately.