How do I defeat my own brain?
Your Brain is a powerful foe that scales its stats with yours, so you can't just level up more and pummel it into submission; you need to be more strategic. These tactics also apply to most scaling monsters.
Improve your initiative. The faster you are, the more likely that you will be able to act first in combat and delay that hit. Spells like Springy Fusilli will help you with this, but there are items and equipment that improve initiative too.
Lower (or at lease don't improve) your Muscle. The stronger you are, the stronger Your Brain will be (both in physical attack defense and HP) and the harder it will hit. Conversely, if you lower your muscle, Your Brain will grow weaker and have less HP, until it hits a minimum level. Unfortunately your HP will also be lower, making this a riskier strategy. If you still want to attempt this, some equipment and items will penalize your muscle.
Reduce the Monster Level (ML). All monsters' internal statistics are governed by a Monster Level. Increasing the monster level boosts their stats, reducing it decreases those same stats. There are equipment, skills, and combat items that modify the monster level. In this case, you want to lower the Monster Level, so use skills that say that they "weaken monsters". Combat items that reduce ML (also called "delevellers") are also very helpful here. Some delevellers reduce ML by a fixed amount, others by a percentage; in general you want to use the percentage based ones before any fixed ones for better results.
Use Mysticality Skills (especially at high levels). At lower levels, Mysticality, Moxie and Muscle skills hit for roughly the same amount, and improve somewhat as you improve your states but almost all of these skills have caps that limit the maximum damage output. At high level, certain Mysticality based skills are uncapped, so their damage goes up as your mystically continues to improve, without limit. The easiest skill to get that works this way is the Sauceror skill Saucegeyser, but there are other uncapped spells on the wiki. At lower levels, you probably won't have enough Mysticality to hit the cap, so it's best to go with the highest level attack skill that you have.
Use damaging items. Some combat items hurt Monsters. You can use these to damage Your Brain even if you can't hit its stats. The wiki has a complete list of damaging combat items.
Improve damage (without improving stats). Certain items and equipment will improve your spell damage or weapon damage. On a scaling monster like Your Brain, increased weapon damage is not as useful because it will only apply if you can actually hit the monster, which is difficult because it scales. Spell damage has no such problems because spells hit scaling monsters just fine.