Like the man said, at your end you could board at either the big station at Santa Fe and Mineral or the one right next to ACC.
At the north end, the Auraria West and Invesco Field(l (I don't think they've changed the name) stations are both within a few minutes' walk.
Depending on how big the event is, RTD security may be checking tickets on every single person getting off a train at Invesco and writing up people who don't have them. I'd go through Auraria West just to avoid that entirely.
Alternately, the Route 31 bus lets off on the other side of the stadium. Its south end is somewhere in Littleton. Slower but might be less crowded.
ETA: the parking lot at Mineral is huge and Littleton PD at least used to hammer it pretty heavily.