Virupa was never weak.
It was just that his strength was always shrouded in a shadow of Shiva's other summons.
Virupa himself was more than capable of defeating any average climbers by himself.
But when we consider the stander of Other Shiva's summons he was indeed lacking.
Both in skills and Talents.
Be it against Kapali, Who was extremely talented in sword and had the body structure of the epic,
Ugram, who was literally the descendent of the primordial dragon,
or the recently joined Karna, who became an Umbra.
let's not talk about Bhima here, since only a little information about has been known to even us.
Compared to all of them, Virupa was indeed lacking.
But it was not his mistake, he worked just as hard as everyone, he practiced as hard as Kapali, was as obedient as Karna and he was more determined than Ugram.
But the limit of birth was not something he can do something about.
His Origin itself was humble.
1. Shadows of Vice 2. Healer's Awakening: Beyond the Game