The shield didn't have much of an impact there, as there were only a few options: a shield on the angels or a bless on the archers (who had already fired). It wasn't possible to cast mass haste, as Maretti had cast it last, and an oil spell could follow. Not casting it wasn't an option either, as Maretti could kill the phoenix while the pegasus was moving, depriving Stinger of a cast.

The main mistake, in my opinion, was not using all the resources that were available at the beginning of the battle. If there had been more money, it would have been possible to buy out after the second mass haste on the crusaders' turn, saving more than half of the army (3 angels, all marksmen, and the Pokémon), while Maretti would have already lost more than half of his troops (only wood elementals and some skeletons would remain). The second battle would have been won easily - there's no speed, no grand creatures, the marksmen are shooting everything, and Maretti would probably have bought out immediately after getting his turn, and then who knows how things would have gone.