In the fifth level of the first pack, there is a large cave with spikes and a body of water with a coin in it.
Go straight ahead until the edge and then jump while going right. Make sure not to jump too early. Then when you are high enough go left. Do it twice. Then keep going left and jump on the spikes then right. Keeping going until you reach the edge. Go right and quickly keep going left. You will get into the portal. Additionally, at the beginning, there is a coin in the water that you might want to get.
Getting killed by spikes, drowning, falling off are all risks.