The rex champ is tough.

But... Is he IMPOSSIBLE?
Nah, But hes Tough.

In my guide i will give you some advice to help you beat this short armed freak into the ground, with a short commentary-less video just in case you are a visual learner.

:LEVEL:

Fist and foremost i would not attempt to fight REX CHAMP unless you are over level 40.
Its an idle game, so just tell your little hero to get to work while you go grind levels in Shin Megami Tensei V. Your yolk hero has got this.

In the included video I am level 54 when beating REX CHAMP, But i was closer to level 45 on my first clear.

:GEAR:

Gear obviously varies by class, but we live in a capitalist society; so buy the most expensive gear, its the best. It will include special abilities that will make the fight easier.

In my playthrough, my character LILY the 4th, is a druid.

My weapon was the sages staff, which has the smash ability, which is a free hit on your opponent. this attack usually hits for max (999) damage to all other opponents, but hits rex for less. hes that tough.

My armor was the thorn robe, with the debuff skill. it lowers the enemys stats. obviously this will come in handy against the toughest boss in the game.

Your gear may be different, but as long as you have the best available to you there will be a massive benefit.

:ITEMS:

Heres the real meat and potatoes. You are probably like me, and mostly ignored items for 90% of Yolk Heroes. Well dust the ole shop off, because these things are going to take us over the finish line.

You will need to have at least one of each of the following items.
Max Ether (increases the amount of times your ability will be used)
STR+++ (stat boost)
DEX+++ (stat boost)
INT+++ (stat boost)
Magic Rope (takes you directly to REX CHAMP, preventing random encounters)

You should aslo buy nine MAX POTIONS. Max potions heal you to the MAX... obviously. REX CHAMP hits HARD, so you want hard healing. If you are under level 50, i recommend buying more healing items like burgers, as you may need the extra help in the very end. Better safe than sorry.

Here we are. Its time to fight.

I play this game 50% of the time on my steam deck, but if you have the option i would recommend playing on a desktop or with a mouse. it will make your life easier. You can hover your mouse cursor over the spot that your potions will be in the menu, so when you hit TAB to open your inventory a potion is a click away.

In Chapter four, open your map and start the "The Real Rat Champ HQ" quest.

This is it.

As soon as the quest starts and you navigate the dialog, use your items.

Max Ether
STR+++
DEX+++
INT+++

and Finally, use your magic rope LAST, as it will take you directly to REX CHAMP.

Once the Fight starts, its literally a slap fight until one of you dies. No gimmicks, no extra targets, Nothing but you and the REX himself in a ring. Mano e Mano.

No Idling this one, This is a Full out fight that you must be engaged in.
Watch the video, I follow a cadence that may be easier to understand visually insetad of reading: but essentially you want to:

Hit all attack prompts, no matter what.
Hit all defense prompts, as much as you can.
Heal whenever you go near or below a THIRD of your health.

REX can not damage you while you are attacking, OR before or during your defense. So the best time to pop your potions is after you attack and before you defend. Its very easy to do if you hover your mouse over Max potion and slap TAB between prompts. Just watch the video and Do your best. You can do it.

It will get harder if you run out of max potions and have to use crappier potions to stay alive, but momma didnt raise no coward. You can beat REX.