The best answer depends on the sun angle and how much of the scene is shaded versus sunlit. Under trees, deep shade can be difficult: it may be much darker and can add an unpleasant green color cast from surrounding foliage.

A good approach is to place your subject in open shade near the edge of the tree’s drip line rather than deep under the canopy or against the trunk. That usually gives cleaner, more flattering light.

For fill, you may only need a small reflector, but if the background or ambient light is much brighter, stronger flash may be necessary. The exact modifier isn’t fixed by the settings alone.

If you want to shoot at f/2.8 and ISO 400 in bright outdoor conditions, you may need a neutral density filter to avoid exceeding your camera’s maximum shutter speed, especially outside dawn or dusk.