When the alien shows up beginning at level eleven, he will steal all of your experience unless you trade your weed for experience. Save up a ton of weed and then trade with him for a fairly quick way to level up, because he will trade you at a ratio of 1 exp for 3 weed. Use the time lapse cheat as previously mentioned (setting the time ahead on your phone) to grow a massive amount of weed to trade to him.
Otherwise, if you want to keep your weed and get rid of him while keeping your experience points, buy the painting from Mary (after smoking her out, so that you get it for 50% off the original price). Tell him to piss off when he comes to the door and he will leave your weed alone, but he will steal the painting.
If you don’t have the painting but you don’t have much weed either, just let him have the weed. The alien will never turn down any amount of weed even if you have just one. If you have literally zero weed left, let him hang out at the door for a bit, plant some weed, then do the time lapse cheat to harvest it quickly (he will still be there) and give it to him.
Want to gain an experience level immediately? When the alien comes up, tap on “Warp to the next level” and you will have to pay $5.00; however, you will gain an experience level automatically. If you are going to use this, use this right after gaining a level, when your experience is near zero, not when you are already somewhat close to gaining an experience level on your own.
If you do the time lapse cheat, you will speed up your weed growth, but you won’t slow down the playing of whatever vinyl is on the turntables. Grow weed, speed it up, then play a vinyl right before you harvest it. Harvest it all, then immediately plant more, water it and do the time lapse cheat again. Harvest it again while the music is still playing. Repeat until the music stops playing, and you can sometimes get three full harvests out of one single vinyl record.