"ぅゎょぅι゛ょっょぃ" is used mostly on extremely strong, loli-type characters (depending on some people, it evokes the feeling of "you don't wanna mess with this girl").
The tag is seen written in the small version of hiragana, with its literation marks written separately in order to create an impression of someone mumbling. However, written it normally as "うわようじょつよい" is fine as it brings larger impact.
The term comes from 2ch, where the quote was originally written as:
1 水先案名無い人 sage 03/06/12 23:28 ID:9RHgfeuk
"ぅゎょぅι゛ょっょぃ" is probably the derivative version of the original sentence.
↑ Optional, hidden boss from Star Ocean 3. Strong enough to kill low-level players with just a stab unless the player character level grinds to match her power.