How to Beat the Missions on Heroic Difficulty
Heroic mode in MoWAS2 is intentionally made to be infuriatingly difficult and unfair, the devs also expect players to mainly complete Heroic mode in CO-OP, not solo.
The way to beat solo Heroic missions with 10 flags+ is to treat it like a tower defense game and abuse slow game speed and quicksaves to the fullest.
For some missions you HAVE to cap the first flag or two as soon as possible in order to get some better reinforcement options (mainly tanks) before the endless hordes of AI spam begin, or else you’re screwed.
Buy snipers, mortars and (when available) flamethrowers, you’ll rely on those for 90% of the killing early-game, then you must switch to doing 90% of the kills with tanks and HE spam.
First have snipers clear flags and kill AT gun crews. Use your mortars for clearing out masses of infantry and keep your flamethrowers as desperate close-quarters defense in case the hordes of enemy infantry get too close, but also if you find an opportunity to use a flamethrower to clear an objective, use it.
Other than that, it’s all trial & error. Bash your head against the wall enough times until you figure out a winning strategy. Beating most Heroic missions on your 1st or even 2nd try is nearly impossible.