Shield needs to have 2 gem slots. Fill one slot with a metal gem and one with a beam gem. When projectiles hit this variation of the shield they get added to it in the form of ‘cells’. There are 5 in the shield normally and it looked to me like I only needed an additional 15 for this trophy making the total 20, rather than adding 20 to the basic shield.
The cells that are added disappear over time and disappear if you hit anything with the shield (it also works as a weapon and kills the enemies you’re trying to use for this if they hit it). The shield is quite likely to hit things that come close since the cells get added wherever they hit instead of in sensible rows. It may be sticking right out in front of you.
The best way to get this is with either a large volume of enemies that back off and fire projectiles, or a few enemies that fire large volumes of projectiles. I got it in the cestus campaign with a couple enemies on screen who don’t leave planet surfaces and fire a ton of rockets. Since they can’t fly into the shield you can just back off a little and let them fire at you. One of these enemies wasn’t enough since cells time out between the enemy attacks but I managed with just 2 I believe.