A friend is going to the Thailand (Tennis) Open this weekend at the Impact Arena. He's been driving but I will ask him what it's like traffic-wise or if he knows anything about shuttle buses, when I see him tomorrow at work.

It looks like there is a bus (No. 166) from Victory Monument to Impact Arena, which runs on the Expressway so that might be an option. (Catching a bus at VM can be challenging too; you'll have to walk around the various waiting areas to find that number displayed. My WAG is that it could be on the northeast quadrant as that would allow nearly direct access to the expressway, but that may be too logical?)

Catching a bus at Mo Chit BTS station can be challenging as there are hundreds of busses, and they hardly stop, so you have to be ready to run and jump on as they slow. I don't ever recall seeing an Impact Arena shuttle bus at Mo Chit, but then I was catching a bus every day at ~ 08:00 so they may not have been running at that time.

Maybe it would be best to take a taxi the first time/day, then scout out return transport to the BTS or MRT at the Impact Arena for subsequent use.

The shuttle bus service is detailed here http://www.impact.co.th/bus.php

The picture of the bus should help finding it at Mo Chit.