Alright since I watch a lot of Ultraman show I'll explain about concept manip they did
simple they not attack at conceptual level but they can attack or interact concept thing
well here more example they can harm and interact with spiritual being but they not attack directly at your soul if you have physical body
due to their inner light that make them can interact with a lot of thing that isn't relate to physical
such as
Mebius just charge himself at those poor gold dudes and close the portal
or
Zero save mirror knight by using his light to purify darkness
or
Ultraman Z prevent the collapse by just beam til it gone
they not type of character who just. . . haha you have soul
demolish your souls no they not right that but still can do it if situation demand
I'll vote for ace for now the reason is Ace will have regular fight while Isshiki have to fight while don't know his own technique might end up kill himself since both know each other ability isn't on this condition list
like
Parasite Technique if he enter ace souls he just going to get blast into oblivion by inner light
or
Sukunahikona that everyone was talking about it either get ignore completely or still work fine let assume it work so what the problem ?
some of ace attack is homing projectile mean if that beam isn't get destroy or be block it will toward himself the moment it exit Sukunahikona
like his signature
Ultra Guillotine that can multiply itself and move in non direct line forcing him has to looking at multiple angle
and has
same start up pose as
Guillotine Shot
while ace also resist some of his abilities and have
shit load of abilities to mess with him (if you want to know what ace can no Ultraman wiki is much better just scroll down and you will see), have more experience in fact true from Isshiki is less dangerous than
Yapool Ace's Rival who have similar power and abilities but much better
so for Isshiki to Ace is just worse version of Yapool,