If you good idea in what it did it context here a image with Pokemon:
He used a masterball on a common Pokemon.
The potion he used on Renka (Amrita) worth at least 100,000,000 ¥, and and the price to resurrect Renka would have been less than half that price.
I was talking about Renka saying people wouldn't combine the two because of her cards bad reputation, when in actuality people wouldn't combine the Amrita with any card since there is usually a more cost effective solution. Why use a full restore when a super potion works? But that was the only healing item he had left and was convinced to take the gamble.
We haven't really seen any mention of existence of a stronger potion in the series yet.
Also no mention of how the Cards self-recover from serious injuries, or how long it takes them to self-heal.
Either way, without Amrita, MC had no chance of healing Renka so quickly, as every other known method would have required him to withdraw from the competition.
In Pokemon terms, MC would be considered a new trainer who doesn't have access to strong healing potion, but did get access to one Full Restore which he got through an event.
Also just to note, the bad reputation isn't particularly related to Renka's card (aka., Zashiki-warashi card), but rather to her two Acquired Skills "Fallen Existence" & "Locked Heart".
Many cards apparently have those skills depending on their interaction (or no interaction) with their former masters, as those are Acquired Skills, aka., those skills are unique to the cards themselves (but multiple cards can have the same skill which is why they are known).
Also Acquired Skills can go through change, like Yuuki's Acquired Skill "Cowardice" (which on any card would make it a Fail Card) got changed to "Little Hero", but the condition for changing bad aquired skill vary depending on the card and their individual characters.
The main point is that, people usually only use the Fail Cards as backup for ranking up or reviving their primary cards or using them as spare combat force. Very few people actually try to use Fail Cards, and even more fewer people would have access to rare items like Amrita.
Aka., the chance of a person having both a Bad Skill card and Amrita is pretty low. Add to that the high selling cost of Amrita as it is a high demand item because of its rejuvenating properties on people.
This makes it almost impossible for usage of them unless the person in question is in a desperate situation were they have no other choice (normally any other person would have withdrawn from the competition upon his card receiving such damage, rather than wasting an Amrita).