What is the meaning of affinity” in this context?

Let's check the meaning of affinity according to the Oxford Dictionary

affinity

A natural liking for and understanding of someone or something.

[Biochemistry] The degree to which a substance tends to combine with another

And let's check also the thesaurus

affinity

correspondence, relationship, association, link, analogy, similitude, agreement, compatibility, congruity, parallelism, consonance

You can not translate it as "critical chance". Notice the two synonyms that I've highlighted: compatibility and consonance. Affinity in your context marks the degree of compatibility between a weapon and a monster, meaning that the most compatible weapon is the one that causes more harm - critical hits - to the monster.

Notice the second nuance of affinity: degree to which a substance tends to combine with another. What about the degree to which a weapon combines with a monster? That's affinity.

So I think that it's perfectly idiomatic the usage of affinity in your context. Maybe another good synonym for your context is fitness.

fitness for/to do The quality of being suitable to fulfil a particular role or task.

being that role "killing a specific monster".

UPDATED: OP has indicated that the affinity is the same independently of the kind of monster. Still it got nothing to do with the Japanese gaming culture or a mistranslation. It's a term derived from the original role playing games. If the term is not related to the monsters, probably it applies to your chosen character. For example if you choose an archer, your chances of striking a critical hit with a bow should be greater than the ones that you get using a sword. Affinity is the grade of compatibility between the character and a specific kind of weapon.

This shows grade level based on the word's complexity.

/ əˈfɪn ɪ ti /

noun, plural af·fin·i·ties.

a natural liking for or attraction to a person, thing, idea, etc.

a person, thing, idea, etc., for which such a natural liking or attraction is felt.

relationship by marriage or by ties other than those of blood (distinguished from consanguinity).

inherent likeness or agreement; close resemblance or connection.

Biology. the phylogenetic relationship between two organisms or groups of organisms resulting in a resemblance in general plan or structure, or in the essential structural parts.

Chemistry. the force by which atoms are held together in chemical compounds.

of or relating to persons who share the same interests: to arrange charter flights for opera lovers and other affinity groups.

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1275–1325; Middle English affinite<Middle French <Latin affīnitās connection by marriage. See affine, -ity

non·af·fin·i·ty, noun, plural non·af·fin·i·ties, adjective

affine, affined, affine geometry, affine group, affinitive, affinity, affinity card, affinity fraud, affinity group, affirm, affirmance

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022

affection, closeness, fondness, leaning, rapport, sympathy, weakness, kinship, resemblance, attraction, compatibility, cotton, druthers, partiality, thing, alikeness, alliance, analogy, association, connection

Then I went to San Diego and started my first show at 21 at One America News, which has recently gotten a little bit more popular and some more headlines around it, of course, given the president’s affinity for it.
If affinity audiences are floating users whose attention towards your products may falter at times, custom affinity audiences are anchored to your company.
All those who place ads on Google are allowed to address their affinity audiences through Gmail, video, display, and search efforts.
This effect “was driven by their intolerance for incongruent comments rather than an elevated affinity for congruent comments,” the researchers wrote.

While this type of content doesn’t yield a huge amount of income in the short term, it does help to build brand affinity, and ultimately sales, in the long term.
He can use that affinity to build confidence for a Jacksonian approach to world chaos.
That affinity has to come from somewhere besides just the entertainment value.
DJ Steve Aoki: To Cake or Not To Cake|Steve Aoki|August 8, 2014|DAILY BEAST
How did it come to be then that she would feel an affinity for Aurora and start to care for her?
Where does your affinity for long, tracking shots come from?
Jokes aside, the folksy, blunt-talking Republican had a real affinity and passion for the issue.
Is there to be some mysterious affinity between chewing and the revolutions, especially the social revolutions of the future?
I don't doubt you will find more than one affinity if you are awakening; that is merely the mating instinct.
Ancestors|Gertrude Atherton
The striking resemblance of Kingia, in caudex and leaves, to Xanthorrhoea, cannot fail to suggest its affinity to that genus also.
Yet we are told that heat, motion, electricity and chemical affinity are the causes of mental and moral action.
Gospel Philosophy|J. H. Ward
In none of these particulars does man stand quite alone; in all of them an affinity with the lower animals exists.
Man And His Ancestor|Charles Morris

(foll by with or for) a natural liking, taste, or inclination towards a person or thing

the person or thing so liked

a close similarity in appearance or quality; inherent likeness

relationship by marriage or by ties other than of blood, as by adoptionCompare consanguinity

similarity in structure, form, etc, between different animals, plants, or languages

chem

the tendency for two substances to combine; chemical attraction
a measure of the tendency of a chemical reaction to take place expressed in terms of the free energy changeSymbol: A

biology a measure of the degree of interaction between two molecules, such as an antigen and antibody or a hormone and its receptor

C14: via Old French from Latin affīnitāt- connected by marriage, from affīnis bordering on, related

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

An attraction or force between particles that causes them to combine.

The attraction between an antigen and an antibody.

A relationship or resemblance in structure between species that suggests a common origin.

The selective staining of a tissue by a dye.

The selective uptake of a dye, chemical, or other substance by a tissue.

A relationship or resemblance in structure between species that suggests a common origin.

An attraction or force between particles that causes them to combine, as the attraction between an antigen and an antibody.

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