In a game of Assault the attacking team has to take two control points on the map (the second only opening up after you've taken the first) and the defending team have to stop this happening. Assault takes place on the Hanamura, Temple of Anubis, and Volskaya Industries maps, and they all have some nasty choke points and flanking routes for attackers and defenders alike. Use this guide to get some tactics and tips, as well as the low down on some good heroes to choose.
If you're attacking then the key is to move fast but stay wary, because the enemy will have had time to prepare their defenses. If they're smart that means setting up turrets and maybe even a secondary position well ahead of the control point, so you should take that into account and use a team that combines shields and defense with movement. You need to take the first control point as quickly as possible, so you have more time for the final assault. When you've broken through and taken the first point you can then switch to a more damage–focused style.
Remember that the second control point will be much harder to capture than the first, so don't all come from the same direction at once; try sending some players in to flank the enemy while one or two of your team are distracting them. If you just keep smashing into their defenses then change your tactics, because doing the same thing over and over again but expecting a different result isn't going to end well for you.
We've found Pharah is probably the most useful offense hero to play here, especially for the first control point. Her primary attack is ranged, as well as having a fair amount of splash damage from the explosion, so it can clear a turret or scatter a group of enemies whilst keeping you (relatively) out of danger. Once you get to the second control point it's often the case that faster heroes like Tracer are more helpful, because you'll be so close to the enemy respawn point. Reaper is also good for his teleporting abilities, because, like Tracer, you use him to get closer to the control point whilst avoiding damage.
Generally speaking for an attacking team on assault you're not going to need to focus on the defensive heroes that much. That said, it's likely that the enemy will be dug in in some hard to reach places and with ranged attacks, like Torbjorn's turrets, or a Bastion planted for maximum damage. If that's the case you can use the two snipers, Hanzo and Widowmaker, to take them out and clear the way for the rest of the team – Widowmaker can also use her abilities to give you all a heads up on the whereabouts of the enemy, making it easier to plan ahead.
Mei and her ice walls are also a good option, because they can block all the damage and allow your team to get past or overwhelm the player giving you problems (her primary fire can also freeze Torbjorn's turrets)
A tank like Zarya is great because you can shield members of your team whilst keeping everyone mobile and not having to clump together. Roadhog is good if you've broken the enemy's defensive line and are going on an all out assault to deal some damage (especially as he can heal himself, so doesn't need as much support). You can also use Reinhardt to bring members of your squad up to the control point whilst keeping them under the cover of your shield.
Lucio is a great choice because he can buff the speed of your entire team and get everyone up to the control point quickly. Depending on how entrenched the enemy are you might want to use Zenyatta, whose mix of support and offensive powers can be a real boost to the team.
If you set up properly then there's no reason a team playing defense on Assault shouldn't win every time (setting up properly should here be read as "actually putting up defenses and not
running up to taunt the opposing team while the timer runs out"). Make sure your team has at least one builder and one Bastion, this being a good recipe for success, and set up turrets in intelligent places – anywhere hard to reach for the other team, or where they won't be expecting it. Get creative.
For the first control point you want to burn out the time as much as possible, so slowing down the team can be just as useful as killing them (although killing them is probably the surest way to slow them down. The second control point is usually much easier to defend, since it's much closer to your respawn point. Setting up a couple of Bastions on the point with some general support is a lazy, but often effective, way to win, especially if you're up against a weak team.
For defense you're probably going to get more mileage out of faster, more mobile offense characters, like Genji, Tracer, or Reaper who can all move around the enemy and get up to hard–to–reach positions, but you can also get some great use out of McCree and his flashbang to stall advancing enemy heroes.
Torbjorn and Bastion can stop an entire attacking team in their tracks if you use them properly. Torbjorn's turrets and Bastion's secondary firing mode can tear through a tank's shield and cut down the attacking team (the turrets especially, since you don't have to worry about human error creeping in on their aiming).
Junkrat is also capable of taking out an entire team by using his ultimate ability, especially if they're all clustered on the control point, and his traps are great for snaring the unwary, especially if they're making a dash for the control.
Since your emphasis is on stopping the other team you won't need an aggressive tank like Roadhog. Winston can shield turret banks you have set up, or anyone grouped in the control point (plus his ultimate ability is pretty intimidating, and can scatter the enemy away from the objective).
D.Va is likewise good at scattering teams with her boosters. She can also lay down some continuous fire with her mech's weapons, because they don't need to reload, which is good for keeping enemies suppressed and off the control point.
Mercy is great if your team have their defenses set up well; she can buff for damage on high–attack characters as well as healing key defensive players like Torbjorn and any tanks on the field. The other useful support hero is Symmetra, who can block an enemy advance with clusters of her sentry turrets. The other two support heroes aren't as useful to a defending team, because their emphasis is more on movement and attack support.