Even his in game boss forms should be able to beat Kharg. I wasn't talking about the super boss form.
Possible things that could have happened
1. You didn't build Kharg properly so he seems weaker
2. You were underleveled for Zed
3. You didn't twink properly for Zed
4. Kharg has moves that you don't allow that others do
5. You scale bosses different
6. You allow different gear than others
Probably a lot more that are slipping my mind. Basic idea is that everyone sees people differently. I haven't played either of these two games, so I can't help you there, but some examples for these.
1. Not getting Tidus Quick Hit for some reason. You would not understand the "speed game" that people always talk about and you'd think Tidus is trash.
2. You go up against Teepo in level 20. You're going to see Teepo as a high godlike where most see him as a low godlike/decent heavy.
3. Equiping all Ice gear for Rubicant will also make him have some obscene damage (I think, never tried it.)
4. Some people allow Terra Ultima, others don't. This drastically changes her power, especially against people who resist fire.
5. You might not give bosses as big of an HP hit as other people do. People range Ghaleon from like 5-12x PCHP
6. Terra again. Allowing/Not Allowing Minerva is going to change her power quite a bit.