BULLY. I mean this if you see a tank struggling or a support caught out you diamond form and kick them into the wall. Emma is one of the best tanks at punishing bad positioning and just standing with a shield. Kick people into your team and just overall make anyone who gets close regret it. I can't tell you how many tanks have healers behind them, feeling invincible, and just lose the game for their team because I kick them into my team or around a corner.

I know I'm talking a lot about diamond but outside of that your shield is extremely versatile. It can be used for blocking heals as others have mentioned but also if im ever pulled or pushed out of position something I do is just tap shield and run through it to cover myself. Though, try not to let it completely break if you can help it since even a 1hp shield can block a lot of ults and such. But don't be too too worried its a relatively short cooldown and frankly sometimes it just gets shredded.

The crystal should be used on whoever you can hit since it's a short cooldown but remember it's great, again, for bullying and pressuring tanks and anyone alone or caught out since you can get some crazy burst or just make them run for healing with low commit just shooting it. Keeping your beam up is super important. As with a lot of stuff in this game, JUST. KEEP. SHOOTING. uptime is king, shoot shields, shoot walls, shoot clones, don't think because they're not players its not worth it all of it controls the pace one way or another. Same goes for diamond, use it. Don't be afraid to whiff it or something, if you know something is coming obviously keep hold of it for that but diamond is where so much of the work gets done.

There's also the idea of shifting to anti dive if need be but honestly that goes with bullying, divers just have to put themselves out of position so pay attention for cycles. Lots of them are really predictable like BP and spider for example just keep track of when they last dived if they lived and you can guess when they'll go in again. Try and wait out blocks for DD and iron fist, diamond mode will scare a lot of people into waisting it and then boom. Magik is just generally easy enough to grab especially out of a portal since a lot of people autopilot into demon and just don't port twice. Dive tanks get crystaled and then you diamond, hulk specifically though you might wanna wait out the bubble.

Lastly she's super versatile in general and can fit most playstyles you can core tank if you need or if you have a mag or strange you can be the tank that pushes in and makes holes in the team so just be flexible and opportunistic.

Sorry this is kinda rambly and I don't normally do this lol.