I found a flaw in the map 'Keystone' by blarg, on Bomberball-hold mode with the Resize mutation.
On this map there is one corner room on the upper-floor with a grated floor. Grab the ball and a weapon with ricochet damage. Go to the room with the grated floor and fire downwards to ricochet back at you, damaging you and resizing you small enough to fit through the grate. (You can switch to 3rd-person view by pressing F9, which helps to judge your size relative to the grate holes.)
Below this grate is a pool of water surrounded by solid walls/floor. Other players need accurate shots in order to hit you under the grate, so they would probably also make themselves smaller to get to you. This gives you an advantage, because they wouldn't have recovered by the time they pass through the grate. Bots aren't smart enough or accurate enough to hit you underneath the grate. In most cases, you can just juggle the ball whilst under the grate until you reach the score limit.
If anyone else finds flaws in the official levels released for Red Eclipse, please post them here.