When I first played through MQ, it had the common hardtype hack problem of being very finicky rather than difficult. Enemies didn't limit your options or press your capabilities so much as they dropped a nuclear bomb on your house for daring to use specific abilities, badges or items in their exact use case. This never actually made any of its challenges difficult to surpass, because there was a huge number of cheesy options left in from the base game. Well, the devs apparently heard me loud and clear. The updates since then have removed or heavily nerfed almost every tool that the player has access to. Master Quest exists firmly in the "beat godzilla with a toothpick" camp of hardtypes than the more modern style of hardtype which usually focuses on increasing enemy dynamism and a higher ratio of risk/reward. Some of the things you'll be dealing with in this hack:

-A boss that destroys every item in your inventory at once and deals a minimum of 6 damage per turn (without Last Stand)
-A boss that cannot fall below 1 HP until it gets it's scripted heal off (why not just make their max HP higher and get rid of the heal???)
-A boss that becomes invisible in response to you daring to use invisibility yourself, which is your only possible means of avoiding an attack that does preposterous amounts of damage
-A miniboss that is so unintuitive that the developers eventually just gave up and wrote in nondiegetic text that outright tells you what the gimmick is, with a line specifically directed at a youtuber who played the hack and, naturally, didn't understand what he was supposed to do
-Numerous bosses that will unconditionally kill you if you drop to 5 HP to subvert "danger cheese"
-Many, many bosses and enemies that can stun you or your partner for 3+ turns and can reapply that stun to further increase the duration (all stun is unavoidable if not blocked, blocking is not possible while stunned)
-Some attacks that can stun you AND your partner and require ludicrously precise timing to guard on both hits, basically impossible on emulator
-A few attacks that punish you for failing a very strict button mashing test by applying a status effect (a huge accessibility issue, on top of just being annoying)
-Almost every enemy in the game has variable delays on some of their attacks to trip up your block timing, Fromsoft style
-A few boss fights were altered to be even easier than the base game with no real justification (Thamz has stated that the fights are in fact easy on purpose but has never given an actual reason for it). It's as if the devs felt like even the players looking for a brutal challenge should be periodically annoyed with the hack.

Oh, and there are two other hacks just like this that are both better. MQ was built off of pro mode as a base but all of MQ's alterations not related to difficulty seem like lateral moves at best, and Dark Star, this hack's successor, is quite literally the same game but superior in every possible regard.

The idea of pushing a really easy game to be as difficult as its base programming will possibly allow is a neat experiment, and MQ takes it as far as I've ever seen anyone go without getting lazy with it but...seriously, just play Dark Star.