The current situation is that:
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Thauma’s swell arm buffs is absolute instead of percentage. Hence the effect becomes weaker as your character gets better gear. (eg. 100 more magic attack when you have 800 matt (increase by 1/8) vs when you have 2000 magic attack (increase by 1/20)). Having that said, Swell left arm gives 300 more magic attack when max + max attribute and many magic classes are currently complaining about R8 mobs having 2k magic defense, so Thauma C3 might come in handy.
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Thauma is taken with linker/wiz3 because of wiz3’s magic missile which can murder monsters that are linked in 1-2 shots. Thauma’s Swell body skill, which is available at class 1 can double monster’s hp, exp* and loot.
The combo that often occurs is: Joint penalty -> Hangman’s knot -> Swell body -> magic missile 1-2x.
Transpose also allows you to be either high INT OR high CON depending on the situation and your stat build.
Thauma is one of those support class that gets weaker the higher level we are past Thauma c3. Most people just take 1 circle in it for swell body and transpose. Some people take circle 2 for reversi (more of a pvp/world boss competitve thing)