If Negan's overconfidence is the target, an OC could play on it by feigning weakness or incompetence. Make him think they're an easy target, and then when he least expects it, strike back hard. For example, the OC could pretend to be a scared survivor begging for mercy, but then when Negan gets close, they use a hidden weapon to attack.
OC characters can exploit Negan's overconfidence by setting up traps that seem too obvious to be real. Since he underestimates them, he may fall for these simple - looking yet deadly traps. For example, an OC could leave a seemingly unguarded supply cache as bait, knowing that Negan would think it's an easy grab without realizing it's a setup.
To exploit Barroth's weakness in Monster Hunter Stories, use water - based attacks. You can equip water - element weapons or use Monsties with water - type skills.
To best exploit Basarios' weaknesses in Monster Hunter Stories, first, equip water - based weapons. These will deal extra damage due to its water weakness. Also, try to stay close to its underbelly when attacking as it's a vulnerable area. And don't forget to use items that can inflict status effects like poison, which can further weaken it during the fight.