How can I get 2X6X12's cut at a slight angle so I can build a level floor?
If the joists are all the same, layout a master on some 1x material then use a pattern maker bit to rout off your pieces (cut off majority of excess with skill saw first). If they are all slightly different use a straight edge or chalk line to establish your line, then cut off with a circular saw. If your floor is really wild scribe the bottom of each piece, then cut with a band or jig saw. Then lay back in place and mark the top line using a laser to create a global level, or measure and and mark off of a reference point if you want flat but not necessarily "level".