Taking it down little by little doesnt work too well - the gun recharge pool doesnt cover what's needed to take down the Dreadnought from the inside. Use a full gun pool, then salvo all twelve Dumbfire missiles (you ARE flying an Excal, arent you?) and follow up with another gun pool salvo, which should have recharged by then. The best way to do this is from the back of the outside, since (at least the game I played - my computer did some screwy things with WC3...) the hangar bay is either shielded with the same system the outside is or doesnt take damage, and the turrets WILL turn to fire on you if youre inside the ship (passing straight through the dreadnouhgt, of course) and the positioning makes it harder to be hit, as well as guaranteeing that all your firepower will be concentrated in the small area, rather than spreading it over three or four sections of shields. Result: one dead Dreadnought, and many many hours of relentless corvette killing practice.
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Build a man a fire and he will be warm for a night. Light a man on fire and he will be warm for the rest of his life.