Definitions of hardcore:
- intensely loyal; die-hard
- stubbornly resistant to improvement or change
- extremely graphic or explicit

Definitions of casual:
- relaxed and unconcerned
- not regular or permanent
- happening by chance; accidental
- without formality of style or manner

Given the above, it seems the way of contrasting the two terms in gaming is to contrast one player's dedication to a gaming content/group with another player's relaxed and unconcerned attitude toward that same gaming content/group, which may or may not lead to irregular or impermanent participation on the part of the latter.

This means that someone who is dedicated to doing Ultimate content in FFXIV is a hardcore Ultimate raider regardless of if they're any good at it. Of course, people who are not good at something can get discouraged and lose their motivation to do it, so most hardcore players are good (or eventually become good) at the thing they are hardcore for, in part, due to their dedication at doing it.

As another example, someone who is dedicated to finishing a long grind is a hardcore grinder regardless of how long it takes, even if they only play one hour a month due to scheduling issues with something else. Though, again, depending on their hardcore level, the grind can become their priority, and that's where "no-lifing" could occur.

For a casual player, someone who is relaxed and unconcerned about a particular aspect of gaming would tend to not be as good at it as they could be and/or they tend to be ok with others not being as good as them at it. That, I think, defines a casual player better when contrasted with a hardcore player who is dedicated/loyal to that same aspect of gaming.

On the other hand, if "not regular or permanent" is the emphasis on defining casual, it would not fit as well as someone can be both casual and hardcore at the same time, like the previous example of someone who is grinding something, but cannot do it as often as they'd like due to scheduling issues, but is still determined to finishing the grind no matter how long it takes.