Send in army 1. Send in army 2 a bit later. Retreat army 1 just before army 2 arrives, but after doing damage.
one dead 30inf 30art army.
Not to say it's not a strong army... But it's hardly "mathematically unbeatable".
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On the subject at hand; annoying, yes. unbalanced? not really. unbalanced against whom? The AI knows how to do it reasonably well, on the other hand you can do it against the AI too.