Maybe you reached the point where 1 Ai just isnt a challange anymore.
There are many people that need at least 2 brutal Ai's and maybe a challenging map to have some difficulty in such games.
So what you can do is:
1. Go for brutal Ai.
2. Choose a very hard map, many people play on Comet Catcher Redux, because it is very big and you need to defend alot of ground.
3. Dont use the "easy way" when you say you used gs, you probably had many nimbus. Thats probably the easiest way to beat AI's. Try to avoid units like that. (i personnaly try to not use mass snipers, mass heavy tanks, any strider or superweapon, any porc that goes beyond stinger, Slashers make it also very easy to controll large areas) basically all units that make cost without any effort...
4. You could give the Ai an inactive Ai as a support income.
5. Fight 2 AI's at the same time. They expand faster and you need to counter more different units. Carefull when you fight 3 ai, 1 will be air or gs.
6. Avoid hard eco. only solar and wind?
7. Give the ai an headstart. Start the game and wait 1 minute before you start playing.
8. Try to do certain challanges. Beat brutal ai without any porc on Comet Catcher redux.
9. Do an only glaive game against two Brutals on Melange (24x24 map) thats not easy either.
I guess you got some choice now.