Personally if your stupid enough to charge your whole army down the dragons maw that's your own problem. Part of war is knowing yourself and your enemy. If your enemy has been showing signs of magic mastery (Has little development militarily, lots of spell research, many spell related buildings etc.) You should count on him using his big guns and throw a feint to draw his attack then follow up with your real force... That's basic strategy, it's called deception and is kinda the point of the game. I don't think anything needs fixing.
Depends. In too many games, you can save up enough mana or get a high enough mana regeneration rate that they'd recover fast enough to deal with your main force as quickly as their small force. Likewise, why use your big guns on a feint? A smart player would use their small guns on a feint, saving their big guns for the main force...
Seriously, if the only way for a player who focuses on warfare to beat a player who focuses on magic is for the magic guy to screw up, something is wrong.