Let's take a look at their stats.
Alex is Multi City Block AP, Wesker is Small Building, Large Building if he gets Uroboros. Alex would win here.
Alex's speed is Hypersonic, but so is Wesker. Alex can dodge tank shells, which move at maybe 1,575 m/s, or 5,170 ft/s. Wesker can move and react fast enough to dodge bullets from sniper rifles, which can be up to 3,000 ft/s, and can move so fast he appears to be teleporting. To go that fast, you'd have to go around 17,500 meters per second according to an article I found. Wesker probably wins here, though correct me if I'm wrong.
Alex's durability is Multi City Block, with Wesker being Small Building, Large Building with Uroboros.
As for techniques, Mercer's got many different types to choose from, from shockwaves, claws, hammerfists, arm blade, whipfist, and Devastators. Wesker's got his Samurai Edge handgun, cool shades (as far as I know, they don't mean anything), and an adapted fighting style to suit his abilities. I think he can wall cling too. Alex probably has enough options to combat Wesker.
So in the end, seems like the Blacklight virus triumphs over the Progenitor. Wesker's only going to be consumed by the blacklight in the end.
