My guarding skill is only 200 since my MNK dinged lvl90 last month. I afk skillup guarding skill as MNK/DNC in abyssea tahrongi with atma of MC, Vicissitude and Sea Daugther. I use relic knuckles and pdt -30 in gears. Then I set two rules in autoexec for self curing: when TP reach 300% and when my HP drop to 80-85%. Pull 6-8 mandy and try to position them in a way that only one mob is behind me. Engage that mob and go afk. You can try to pull 3-4 mobs for a start and pull more mobs once you are confident that you will not die when afking. Skillup rate is not as fast as MNK/RDM as you cannot pull too many mobs but you can go afk as MNK/DNC

Using a similar method, I cap my parrying skill as SAM/DNC while afking.