Well, here are the contents of the CNS file for Sigma from MMX1, maybe if I can figure or be told how to tweak this I can do the others myself, I was going to post just the nothitby/hitoverride parts but maybe they could be confusing out of their original context so I'm pasting the whole CNS file.
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What I would intend to achieve is to allow a small time window from the first hit to when invincibility kicks in, possibly 1 or 2 seconds, I have been trying to tweak values but when playtesting it still only akes a single hit from any attack and goes invincible. Help is very much appreciated. This file is intact save for the attack lowered to 70 which might be a temporary solution, if Sigma can be hit more times then maybe his damage could be restored to normal and still be defeatable despite being a boss. The only other change I made was to turn the music related states to comment because they override any other background music.