In the 1930s the Red Army has yet to do anything that would impress the Japanese into believing they were invincible.
Actually, the Japanese were scared shitless, because they had a fairly good idea of how the USSR was building up.
From the battle of Khalkin-Gol we can observe that Soviet troops under the best Soviet commander of his generation (Zhukov) can indeed defeat the Japanese in open terrain.
Mrm. A lot of recent scholarship suggests Zhukov was average; check out Thunder in the East, if you're curious about the more recent view of the Soviet military machine.
Another myth is the Red Army being far stronger pre-purge. They were certainly weakened to some extent but even pre-purge the Red Army was a mess. Most of the high ranking officers purged were political leaders anyways.
Cite?