[Kitalrez]

In my experience what you need to look for are mobs that check High Def. Those are the ones you're going to get Skill Ups on. It seems that I rarely get Skill ups on mobs that don't check High Def, and almost never get in on Low Def mobs.
This is the main reason that people Skill Up on Crab type mobs due to the DEF UP TP moves they have to make sure they stay High Def mobs.
Depending on your actual job level, and your weapon level, you can take a few paths to catch up a weapon solo. The higher you are, the better you have Skilling since you have much higher survivability.

Nah, you're misinformed here. High DEF isn't what you're looking for. High EVASION is the magic signal that a mob is good for skilling up. The evasion check is directly related to your accuracy, which is a function of your weapon skill. A good rule of thumb is if anything checks as low evasion, it's probably not going to give you any skillups. Now, if you're say a RNG or DRG with acc bonus traits, and have dual sniper rings and ACC gear up the wazoo, pretty much everything will check as low evasion, making it really hard to figure out what actually gives skillups. If the above scenario doesn't apply to you, then the rule of thumb generally works. The rule of thumb will never beat out a good knowledge of the level charts, however.

The DEF check relates to your attack rating, which is listed on the equipment screen. It just shows whether or not your current attack rating is sufficient to deal decent damage to a particular mob. Attack also rises with weapon skill, but is tied into strength, your weapon's damage rating, etc. The evasion check is pure accuracy, and gives the best idea of what to skillup on.

The reason players like to skillup on crabs isn't because they have high DEF. It's because the crab family generally has pretty lousy evasion. Their attacks are also fairly weak, with Bubble Shower being easily overcome with a Barwater spell, and Big Scissors being very inaccurate. As such, you can afford to hit that crab a lot of times during battle without really suffering too many ill effects. That's the reason players like to skillup on crabs, the longer it takes you to kill something, the more hits you can sneak in, and the more chances you have to gain skillups.

There are mob families out there who do have generally high evasion for their level, I'll leave you to figure out which ones they are. There are also mob types you want to avoid skilling up on, like anything with the MNK job. MNK mobs like Mandragoras have counterattack, which means anyone on the front of them has a chance of being countered. You don't gain skill on a countered attack, and end up taking some damage from it. Generally, also avoid mob families with big AE moves like Scorpions, or nasty breath attacks, like Rams and Wyverns.