These achievements are somewhat stacked and the Supreme AI is not overly difficult. There are 34 skirmish maps, but they are not all available in every mode. Note that you cannot only play Supreme AIs to get these achievements: you must beat at least one Normal AI to get any of the three AI/Skirmish achievements, and at least one Challenge AI to get Holding Your Own and To the Grave. In theory you can play one game with one of each AI and then play all the rest against a single Supreme and get all three achievements, but that is not the most efficient approach. Instead, aim to get the three Skirmish AI achievements plus the Hunter Killer and Supreme Assassin achievements for a total GS of 140.
Set the victory condition to Assassination, Fog of War to None, No Rush time to 20. On on maps that allow 4 players put in a Supreme AI, a Challenge AI and a Normal AI. On maps with 3 players choose a Supreme AI and a Challenge AI and with 2 Players only Supreme AI. Killing all 3 count as 3 ACU or Commander Unit kills towards the Hunter Killer achievement and you get credit for beating the Supreme AI on each map. To kill each commander tech up an air factory to level 3 then make 2 - 3 tech 3 bombers for each commander. Attack the commander units with the bombers and if they kill him all the AI's units die.