Selective herbicide use is easy.

Roundup kills monocots & dicots, thus is NOT selective.
Because you want to kill grass (monocots) among dicots, use an herbicide which only kills monocots.

But realize that bromeliads are monocots. So you could paint the herbicide onto the grass while avoiding the bromeliads.

Check the label to determine whether or not you need to dilute the herbicide for a paint-on application. For that purpose, it's often applied full strength.

Some may be interested in this article.

Here is a link that might be useful: about Glysophates

Shield the bromeliads with something - a paper bag or box will work - and spray the grasses. After the spray dries, remove the shields.

Work systematically from one edge of the are to the other, making sure you don't miss a section.