I could get to level 9 on command. I would usually skip the snake pit. I would always miss the last snake somehow and just try to hit the spikes and land in the hole to go to Stage 7.
I had fun with the Game Genie on it. Found out Level 13 is a thing, and played around in the Terra Tubes, and mastered Clinger Winger (4 lives I believe)
A few times I reached Level 10, but I never made it out legitimate.
"Er...well, y'know. You can't make an omelette without um...destroying a forest.....or something" -Black Mage