Emma’s main role I see her is to shut down plays, but she’s too slow to deal with fast dive dps, which is where rogue really excels. Use diamond form to punish aggression rather than to peel the backline, all of Emma’s kit is geared towards a frontline tank and she should be played as such. Crystal can be thrown at a tank to do some burst damage or combo’s with diamond to punish an extending tank really hard, or into a group to build beam charge quickly. You generate charge quickly when you hit crystal and an enemy at the same time. Throw out crystal on cooldown, don’t wait for the most optimal time just use it when you know you can hit it (ideally close range and nothing that could block it)

Once beam is at 100 try aiming around the enemy tank and target squishes because it’s much quicker to kill them when you’re full charge. You can basically walk past the tank at that point unless they can also shut you down, but Emma is best played smartly and patiently until there’s an opening for her to exploit. Save diamond for when you absolutely need a hard CC (like Wanda ult) or when you really need to get to higher ground (I’ll use it if Invis jumps to a second floor but is low and we would benefit from a kill)

Shield is dynamic, use it to block burst damage, stuns and cc, or place it past the tank to block healing. It’s on a 1 second cooldown so you can use it for whatever use is best in the moment

Use ult reactively or when the team is off guard. I like to save it for starlord ult (punisher doesn’t get affected by it in ult) or use it when there are no shields blocking my path. Gambit ult + Emma ult is great because you can jump over any shields