First set yourself up with 6 hearts as Tommy suggests. I believe you do that by buying them in the main shop with diamonds you pick up. I don't fully recall at this point.
Now that you have your 6 hearts, start whichever zone you choose, but I suggest zone 1 for difficulty purposes. If I recall, all zones mode starts you with 3 hearts and not 6.
Now you're looking for the cracked wall in the level with the portal inside. Specifically you are looking for the dark blue portal. Usually it's in a concrete wall that you need a bomb, or a strong shovel like the pickaxe, blood shovel, or a max multiplier obsidian shovel. Inside there should be a health merchant with some heart containers and food. Don't buy anything yet.
There's a few ways to do this, but your ultimate goal is to kill the shopkeeper without getting hit, and be far enough away when he sets his bomb that it doesn't blow up the food, hearts, or the gluttony charm he drops.
Here's the 2 ways I usually do it.
1) if you have a bomb, and there's 2 empty squares between him and the gold wall, place your bomb to his diagonal top left so the corner will hit him. Now jump up once, and right twice. Your bomb kills him and he sets his bomb 1 square to your right. Jump left now to dodge it, then reap the rewards. Grab the gluttony charm and the food item. Hopefully it's cheese or better, and not an apple, and use it, increasing your hearts by at least 2, then grab the other heart containers to round it off. Hearts cap at 10, any full heart containers after just heal you, empty containers do nothing.
2) same idea, but sit at the bottom middle and throw your dagger at him. Jump left to dodge the bomb after and collect your reward.
If you don't have full health for the gluttony charm, you can just get the heart containers and food first, heal up, and hope to find another food item later to use with the charm.
Hope this helps.
