On a rondom day during the week, the standard Faction Bonus will be replaced with an INVASION event. During this time, the normally disabled Invasion Boss and Invasion Tower is available.
For this achievement, you must score 1000 hits on the boss. However, hits only count after you drop the armor. On the plus side, the hits are cumulative over ALL bosses. You can split it up over many weeks and your total will still run. So if you get fed up (especially on that trick Corrupted Shinnok), you can go away and come back to a more manageable boss. Also to note, if a variation of that boss is giving you too much trouble, go back to player select and a different variation of the boss may appear.
As for combos, Full Auto Jacqui's gun (
), Liu Kang's quick combos, and Thunder God Raiden's quick combo (
hold) can help you get hits. However, that armor is a bitch and you might want to try low attacks to drop armor. Down + front kick is universal for everyone, but check out practice mode to make sure a low attack is not a "Mid" attack.
Frustrating, slow, but not impossible. Grind if you must, but space it out for sanity's sake. There is no tracker available for hits.
BONUS: If you manage to last a full 30 seconds, you'll instantly have your health dropped, but you unlock the Farmer Jax card for the MKX Mobile app.