smaller aparture or low light focusing

[kondath]

Iam trying to use G3 for portrait photography. I use external studio flash fired by slave units.G3 in manual mode I select shutter speed 125 and aparture 8.Even then i get over expose.Is it possible to have a smaller opening say aparture 16,24,32 etc??.With ND filter ON the shots come correctly but due to very low light i cannot focus. any sugetion to focus in very low light or else how to make a smaller aparure ??.Even by reducing flash power it dont work.

regards
Ali