Sentry is vulnerable to Psychic Assault he is fragile psyches and is readily susceptible to high levels mental manipulation.
Sentry once in the The Negative Zone is at his weakest. Once here Robert's powers and health gradually deteriorate and this increases his split personality control known as Void Sentry.
Sentry has average Fighting Skills and he is just a usual brawler where he only knows about Punching and Kicking just as any usual fighter he would be vulnerable to advance fighters such as Black Panther or Thor once in combat.
Sentry lacks full control of his powers and this is one of the fears he has making him more limited and unstable in combat because once he is angry he can't control himself.