There's a reason why your mechanic is warning you not to "cross chain". Listen to him.
If you won't listen to him, listen to Sheldon Brown:
"Try to avoid the gears that make the chain cross over at an extreme angle. These "criss-cross" gears are bad for the chain and sprockets. Especially bad is to combine the inside (small) front sprocket with the outside (small) rear sprocket. This noisy, inefficient gear causes the chain to wear out prematurely."