if you are trying to make a cut for the end of a beam that is visible, a portable band saw works excellent. if you dont have the dough or cant rent one, a skil saw from all four sides and then a sawzall, and then a grinder with a sanding disc to clean it up.
if you are making a cut for a joint, like the top of a post, or a splice, it is a combination of tools. say you are cutting an 8x8 for a post. draw square lines around all four sides, hopefully they all line up when done, if not try again. using a skil saw cut all four sides. then with a chainsaw cut about a half inch longer than needed and cut the piece off. now take the chainsaw in from each side at a slight angle, removing the half inch long part, but do not get the chain into the area already cut. you will be leaving the 2 inches or so around the outside as is and cutting the center of the piece a little bit lower. you can also do the dishing out with a grinder with a sanding disc. cutting the center of the post below the skil saw cuts allows for easy fitting of the pieces. there is still plenty of bearing, but no high spot in the center to open up your joint. i have done a fair amount of structural and architectural timbers and that has always worked well for me.