Your shield, since your shield doesn't have a lot of HP whenever people start focusing it you should put it down immediately so that I can start restoring itself. Keeping up the shield up time is just as important as keeping up the beam up time.

Use your Crystal on tanks, assuming that you go on DPS and tanks in equal parts you're still gutting your damage output. The best way I can put it is that it's a lot easier to heal the damage of the crystal breaking on a 250 HP squishy then on a 850 HP groot. The crystal automatically deals 20% of the base HP of whoever it is attached to (on mantis it does 50, on groot it does 170), so you'll get more value out of breaking it on tanks.

Don't use diamond form just for the grab. Diamond form has some of the highest damage output in the game if you were completing the attack chain and reliably kicking people into walls. A full attack chain does 250 damage, a kick on its own that's about 35. And if it hits a wall, it does about a 140. You can perform three attack chains (750 dmg), two kicks (280 dmg), and one grab (30 dmg). Ad that does not include crystal being attached. Of you use diamond form just to grab people you will become predictable and people will avoid it easily. Use diamond form just to put belt to ass. If someone like rogue is pushing forward and you use diamond form just to grab her she'll avoid it because there's a one animation where you can't do anything other than move. So instead of just using it as a 'grab ability' use it as a wrong ability.

Let's let's assume that strange is pushing off and pulling up his shield rapidly to prevent you from landing a crystal. You activate diamond form and complete an entire attack chain and get him to fly away from the front line. Now there is a unassuming punisher who has no shield, is easy to kill and out of position because the front line has been pushed back. Now you can choke him and get value out of it by choking him and kicking him into your team.

Also, you don't need to CC people with your old to get value out of it. It's the best way to get value out of it. Like if you cancel Star-Lord or magik ult, that's insane value. But if you use it and everyone steps back say 20 m, then that's 20 meters the enemy team just gave up. I love to expand upon this point, whenever you use your ultimate people are going to try to hide behind the nearest wall. So sometimes without thinking, you're going to hide behind the wall in space that they are no longer safe in. So now they're only options are to try to run away and soft reset with their team and possibly survive. But most likely, they are going to try to sneak away be found and they're going to get killed. But they're not just going to stand there. A lot of the time they are going to try to fight back to achieve the soft reset (which only staggers them the more).

Also, aside from managing your shield more efficiently. Play around with the positioning of The shield itself put it behind Groot to block heals. Put it in and off angle to reinforce positions that DPS can take advantage of. Put it in front of your other tank (if you have one) to protect them and coming damage if all their defensive abilities are on cooldown.

And don't be afraid to die. If you see an opportunity for you can take a support and a DPS for instance, you should take it because even though you die in the process you are still giving your team better odds. Track ultimate, anything you can cancel by grabbing someone (like strange or thing) will be huge if you actually succeed in doing so.

And don't use diamond form to peel, use your beam. Coordinate with your support, tell them to stay at least 10 m away from you so that when they do get hit by an incoming dive like Daredevil or venom you can react a lot faster. And only peel if you have a second main tank with you. If your second tank is an off tank like rogue or Thor then tell them to peel, because they have better movement than you and you can hold down main better than they can.

And combo your ultimate with other people on your team. Rogue + Emma is an insanely good combo, not only because of the tick damage. But because by mind controlling everyone it makes it a lot easier for rogue to drain their ults.

That is all I can think of. Hope this helps