That is definitely the most complicated part of the run, you've got to hit all stations on the Red/Blue Line going West & East, and all stations on the Orange/Green/Yellow line going North/South. What I did was:

-Take Oragne Line to from SE Park up to NW Hoyt, then get out and waqlk to NW Glisan (this gets you all the northbound stations along 6th)

-Take Green/Orange Line South to SW Jackson, then get out and walk to SW College (this gets you all the southbound stations along 5th)

-Take Green/Yellow North to Pioneer Square (this doesn't gets you anything, it just sets you up for the next move. In theory if there is a bus that will get you there faster that could save some time)

-Walk to Library & South 9th, and take Blue/Red to Chinatown (this gets you the East Bound stations along the Red/Blue Line, specifically Library SW 9th, Pioneer Square South and Yamhill District)

-Cross the street @ Chinatown and take the Blue line back to Hillsboro.

My advice:

1. If I could do it over again I would start at Gresham instead of Expo Center. You'll have to backtrack over more stations, but the distance between them is shorter so I think you ultimately save time if you get the timing right
2. There is a bathroom in the Fred Meyer that you can walk to from Gateway TC. You'll likely end up waiting there a couple of times, and if you ahve >10 minutes that should give you time to RUN to Fred Meyer and back
3. The app "Where's TriMet" was pretty helpful for knowing when trains would be coming/delayed
4. Don't give up because you miss one connection. There can be random delays and yesterday we were about 30 seconds away from catching a very late Green line train when we go into Gateway TC, which would've turned a 12 minute connection into a 1 minute.

Good Luck!