Easy boosting method:
Host a 5 player custom match with the following settings: min 1 traitor, max 1 traitor, no armory, no serum lab, objective difficulty 50%, only power plant and communications objectives, traitor drop 1 to 1 minute, traitor drop 2 to 1 minute.
At the beginning of the game the group must find the first repair objective, this will trigger the traitor drop which is on a 1 minute timer, as a traitor your objective is to gather a minimum of 4 traitor credits so open the traitor drop and any traitor cashes you find.
Then the group brings the materials to the first objective having the traitor do the repair button (10% of the chievo), then the group must find the second repair objective and mark it with a beacon, here is where the traitor can gain 50% towards this chievo, the group must bring the materials for the second objective, as soon as it is almost finished the traitor must be the one to officially repair it (20% of the chievo), then the traitor can sabotage this second objective and repair it for free 4 more times (60% of the chievo).
Rinse and repeat on more matches.
Why only the second objective and not farm the first? for some reason if the first objective is sabotaged it will require materials for the repair but the second objective is free, but you need at least 4 traitor credits for four sabotages.
Can be done with 2 traitors to maximize the chance of 2 players who might not have the chievo to be traitors, however the second objective can only be sabotaged 4 times regardless of how many traitors there are.