Ok, so you have to take into thought what kind of mental ray lights you need.

First thing- sunlight.
A very strong spot light converted to a mental ray area light (but make sure the cone angle is large) with a high sampling factor (which you find under the mental ray tab in the spot light attributes). As for shadows, use raytraced shadows and just mess around with the values. also, the color needs to be a bit yellowish

Second- ambient light.
You need a bunch of little lights that make light from all around to fake this effect. How about a dome of spotlights, each with an intensity of 0.3, with ray traced shadows, and a lot of trace depth limit.

that should be enough for a nice render of an outdoor object