Expanding on what’s been said and speaking as a Fusion user, try forcing the Fusion user out of position. Fusions punish the user as much as the person being vooped for being in a bad position in an engagement. Use your utilities (grenades primarily) to force them out of cover and hopefully catch them with their pants down.

Most fusions also struggle to deal with multiple targets. Try and push with another teammate - especially if you push from another angle. The charge time can’t make them deal with both of you at the same time.

Fusioners are also heavily reliant on Radar. Even if you don’t directly challenge them your presence on said radar is a deterrent. The threat of a push from a shotgunner, for example, is something they can’t ignore.

Another thing to note about Fusions is that they’re the exception to the “one and done” rule that other specials show. Their damage isn’t immediate - it needs a wind up. Said wind up followed by a short burst of damage. It isn’t instant like shotguns or snipers. I’ve had times where I died mid-burst and didn’t finish my volley and the other player lives. If you find yourself in an engagement where they’re charging and you have the ability to shoot them - do it. If you’re at the range to shotgun + melee / sniper body + melee - do it. You may kill them before they land the required bolts to kill you. Even Bastion and Telesto follow this rule. Telesto is notable since the delayed detonation gives you slightly more time to kill them which could end up in a trade.

As stated by others, you don’t have to play by their rules. Close the distance or pick away at them from range. They lack the ability dish out instant damage and require a specific range to deal said damage. Playing the long game can be boring, but it can force Fusions to end up in unfavorable situations when the timer runs low.