In a multi-player game, the best strategy tends to be steady expansion while constantly upgrading. You expand faster than a turtle and specialize different parts of your empire as needed. You do not expand as fast as a Blitzer though.

A pure blitzer tends to push as far and as fast as possible. When they start losing, they go out quickly though. Turtles are hard to break, but they tend to not expand much. An Expansionist tends to have armies that are the quality of a turtle but with the troop strength of a Blitzer. These tend to be spread across a smaller front that a Blitzer but are able to overwhelm a turlte. Net result is often that Blitzers implode when they fight another Blitzer or they get bogged down cracking a turtle. This is where a steady expansionist tends to do best because they are often very good at popping Blitzers.

The smaller the number of players, the more chance a Blitzer will have to expand fast enough to keep up the momentum. Once that momentum stops, Blitzers tend to go to pieces.

Against an AI, there isn't much to slow a Blitzer down. Throw a dozen turtles around him and you get a much slower expansion. Replace the turtles with expansionists (who will raid like a Blitzer but with a pure goal of weakening their emeny) and you can see a Blitzer go out quickly after expanding fast.

This is often because a Blitzer can't keep a strong enough presence on their borders to kee the expansionists at bay. This becomes more true the larger the Blitzer becomes. It is a dynamic that appears in multi-player games but not in one-on-one games.