if that were true then my fire killer could have lvled her hagendis hit while bursting even though she was a higher lvl because she was getting a lot of exp so I don't think that's true.

You gain proficiency based on the damage you do to the enemy in relation to your lvl. It's like when you start to burst a lvl 12 character and get your skills maxed in less than 10 rounds. But at higher lvls, for example a lvl 180 fighting Cirrus you won't get any proficiency at all. I didn't get soul dodge till my water wit was lvl 180 and no matter how much she uses soul dodge while training she hasn't even made it to 1 proficiency even though she gets about 200 exp from cirrus because she uses freeze and shrink while training.

so a lvl 12 fighting with higher lvl beetles can get proficiency but a lvl 180 fighting with lower lvl cirrus can't get anything proficiency for skills. Based on this I can only say that the proficiency you get towards a skill is based on your lvl in relation to the mobs lvl. If the mob is the same or higher lvl you will gain some proficiency.

As for saying it's based on damage if you look it's easier to lvl attack and hit skills because it focuses your entire attack on 1 mob. The attacks that hit multiple mobs do not. Multi target skills do not take your 1.2k atk and cause damage to each individual mob. It takes your 1.2k atk and divides it amoung the enemies you will damage. That is why multi target attacks seem to cause less damage. Also unlike single target skills, multi target skills can not combo. Meaning less damage is caused to the mob and less proficiency gained.

So in my opinion from what I can tell skills gain lvls based on:
your lvl in relation to the lvl of the mob (mob must be the same or higher lvl)
amount of damage done to a single target

best thing you can do is increase you str/matk as much as you can with equips and find higher lvl mobs that you can fight with and not die.

As for sleep and freeze the more you use it the more proficiency you get. As long as the mobs are around your lvl then it's fine.