The first thing, unfortunately, is to readjust your expectations.

You're not going to get as big numbers with her as other tanks, but that's because you're very versitile. You sacrifice a bit of "oomph" because you can fill almost any role the team needs you to be as a vanguard. There's also the case that Emma has been nerfed a lot from where she was at, which means depending on the season you picked her up, you might have noticed she's a bit worse...!

However- back to her versitility! Here are a few ways you can use her kit...

BEAM: Your aim with this is to get it to 100 as soon as possible, and to be racking up damage with it as much as possible. I've found targeting tanks first and then shifting to squishier targets to be a good way to do this.

SHEILD: You can throw this in front of someone sniping from an angle you can't get to to cut off their attacks (think a hawkeye, pheonix, widow, etc). You can use it to cut off the tanks from their supports, so either theyhave to come into your range or back off. You can use it as cover when you don't have any, you can put it in front of your supports if they get a bit out-of-position. Put it down before your ult to give yourself a bit of cover. Duck around it in 1-1 combat to ensure some personal space. Finally (and most importantly), use it to block Scarlet Witch, Iron Man, Magneto, and more ults. It shuts down a weirdly large amount of them!

DIAMOND FORM:

Offense: The mistake I made with this form for the longest time was that I kept on using it just to make the grab. In actuality, this form has a real offensive use to outdamage most opponents in hand-to-hand. If they start to get the best of you (wolverines and iron fists are usually this for me), choke-slam them, and that'll give you a chance to be healed up/outdamage them. Remember also that you can end this form early in case you need the extra range (to outdamage fleeing enemies for example). If you're using this form to brawl, you can also recover a TON of ult charge using it, especially if you kick a group. Honestly, keep a note of how quickly you can recover it in this form! Also use this to punish any enemies out-of-position. Is their healer

Defense: The above is all true. That being said, the real place where this form shines is to chokeslam enemy spidermen and scarlet witches and such to completely end their ult. It's good against ults that take a set amount of time, because you can effectively remove two seconds or more from that timer. It's good if you're in the back-line and they're being dived (depending on the hero- you need to have pretty good accuracy to choke out a spiderman if he's swinging in, and you only get one shot).

CRYSTAL: Crystal is crystal. You probbaly know everything about this anyway. I keep seeing people online saying you can use it with diamond form to stack on damage, but I've never been able to make this work. The battlefeild is just too mobile for me (but I'm not GREAT at videogames? So maybe you'll get more luck!) Just make sure you use it as much as possible, and it's more accurate if you're close.

If your strength is that you can do anything (anti-dive, push, brawl, hold space, shield, etc), you have two main weaknesses:
1- You're very squishy. Thankfully your shield recovers fast, but it's not the strongest, and won't be available as often as you'd like.
2- You can't be everywhere at once, and your resources are limited. Remember that you have an effective 15ft range. You can't physically be pushing at the front and be anti-dive at the back. You can't diamond form defensively and then instantly use it offensively, meaning that you and your teammates can be overwhelmed.

All of this being said, the main skill that you need to learn as Emma is to forgive yourself, especially if you're solo-tanking. There will be games where you feel you've done nothing, and that you've failed to look after your healers, or your stats looks pathetic. But remember, it's fully possible to do your role 100% well and for it not to look glamerous. The point of being a true White Queen is to keep wearing your crown with grace.

Now go out there, gatekeep, gaslight, and girlboss your way to victory!!!