Muto terram guidelines are as follows for changing earth:
Level 1: Change one property of earth.
Level 2: Change earth to another type of natural earth (sand, mud, loam...) .
Level 3: Change earth so that is slightly unnatural. Requisites may be required.
Change earth into a liquid or gas. Requisites may be required.
Change earth to stone, or vice versa.
Level 4: Change earth so that is highly unnatural. Requisites are often required.
Change earth into a mixture of liquid, solid and gases. Require the appropriate requisites.
Change earth into a plant. Require Herbam requisite.
Make something grow to eight time its previous volume.
Level 5: Change earth into slightly unnatural liquid or gas. Require the appropriate requisite.
Change earth into an animal. Require Animal requisite.
Level 10: Change earth into highly unnatural liquid or gas. Require appropriate requisite.
As I see it base 4 might be enough: transform earth so that it allows you to pass trough it (highly unnatural) and keep structural integrity. No need to change it into a gas or liquid.... You will be dirty when you emerge on the other side, but that is a small price to pay
B4, +1 touch +1 diameter +1 stone = level 15
Transforms 1 cubic pace of stone to an earthy-ish form that allows you to pass through it. That is enough to open a (narrow)doorway in a 1-pace thick stone wall.
More along the lines about what he wants