I mean HL Overwatch gets beat back by one nerd in an orange radiation suit. Imagine what 24 heroes would do to them. Genji looking like Ryu Hayabusa as he soloes the ground troops with his endless swift strike resets and a harmony orb glued to him courtesy of Zenyatta, Pharah making short work of the striders and gunships with infinite rocket spam, the less mobile classes can literally just chill in a fortified position and lay down suppressing fire with Rein, Winston, and D.Va all taking turns using their shielding abilities in a cycle while the others recharge. Widow is popping heads left and right, Mei is creating chokepoints with icewalls, Junkrat is racking up multikills like no tomorrow with endless grenade spam, and so on and so forth until nanoboosted McCree walks out on the rooftop and scores the most epic High Noon of all time. The Citadel would need to be hiding some substantially more powerful tech than what we've seen in HL2 to be up to par. I have a feeling their invasion force was different than the occupation one we currently see at the time of the game.