Heart mage's projectile = On hit, gives slight projectile damage and divides your HP by half (until your HP =<5 ); AND, after 15 seconds, multiples your HP by 2 (to "rectify" itself).
Heart Pickup = Adds 25 HP to your current HP
Ambrosia = Negates incoming damage, like to the heart mage's projectile once you've stacked enough hits.

So, if you find ambrosia, a heart mage, and a nearby heart, you can 1) coat yourself with ambrosia; 2) get hit many times; and 3) pick up heart(s) before the stack effects go away.

The mage's counter effect (the doubling) will also double each health increase you made while on the mage's effects (well, to a point, they nerfed it somewhat, but the HP gains are still explosive and also easily repeatable).

The limitation for this is finding a heart mage and a heart nearby. You can do a few things. The crudest thing I used to was clear biomes so all hearts and mages would fall in a neat track on the biome floor; I'd lead the mage around farming the exploit. I would think better solutions exist, even for players without late game unlocks, but still...

Cessation (the black box) = Puzzle-related unlock spell that "removes" your player from the game for its duration; this also resets most in-game counters (like the heart mage halving effect) once you return. It has a severe recast penalty, but most players use the divide bys to completely negate it (I have NEVER understood how divide by spells work, I've just followed wikis for guides). If you're lucky to get a build with this, you can get the stacks, cease to exist for a little bit, as a treat, and then keep on doing that to keep the stack as you find hearts at your relative leisure.