Now see, what makes sense is...
If Flash has that long to prepare, what he would do is run SUPER-FAST around Doom, checking out his armor and such, looking at it at every angle, seeing where hinges and screws and such are. If you're not following me so far, Flash could do this all without Doom even knowing. That's just how fast he is.
Then when they start fighting, Flash just freakin takes apart his suit. In an instant. I mean, that's just one example.
Another, a cheap way, but entirely able to be utilized, is for Flash to vibrate inside of Doom. Doom explodes. Inside of his armor. Eww...