A remarkable 20 wickets fell on the opening day on Saturday, with WA skittled for 120 before SA were dismissed for 124.
There was even more carnage on Sunday, with Western Australia slumping to 5-13 before being rolled for 66 within the space of a session.
It left South Australia with a meagre victory target of 63, and the visitors rocketed to the total before tea on day two, with Jason Sangha (24 not out) and opener Conor McInerney (23) leading the way.
WA entered the round in fourth spot, and the heavy loss has put a huge dent in their hopes of snaring a fourth consecutive Sheffield Shield crown.
The Warriors only have three more matches to resurrect their season, and they could be fifth by the end of the current round.
Ladder leaders South Australia, with four wins and two draws from six matches, are in the box seat to host the Shield final.
McAndrew only snared one wicket in WA's first innings, but the 31-year-old was unstoppable on Sunday on the way to career-best figures.
The Redbacks' first-innings lead of just four runs didn't seem like much at the time, but WA lost three second-innings wickets before they were able to erase the deficit.
The Warriors were 2-0 and 3-2 as Sam Fanning (0), Teague Wyllie (0) and Jayden Goodwin (two) all fell in the blink of an eye.
Fanning and Wyllie were caught at slip, while Goodwin edged McAndrew through to the wicketkeeper.
Hilton Cartwright (two) was the next to fall when McAndrew pulled off a brilliant, one-handed catch while falling to the ground off his own bowling.
And McAndrew had the remarkable figures of 4-2 from 3.2 overs when Joel Curtis (one) edged him behind in the seventh over of the innings.
WA were 5-13 at that point, and in danger of not passing the lowest ever Shield total - the 27 South Australia posted against NSW in Sydney in 1955.
Ashton Turner (30 off 36 balls) and Keaton Critchell (11 off 52) combined for a 39-run stand to at least steer WA away from setting any all-time lows.
But it was only a momentary reprieve, with McAndrew and Jordan Buckingham (3-27) racing through the middle order and tail to ensure WA didn't even last the session.
WA's poor Shield display continued a worrying trend this summer.
In October, they suffered a collapse of 8-1 in a One Day Cup loss to Tasmania that all but ended their hopes of defending their crown in that format.
