The shield isn't meant for holding a position long-term, it's more to give you a bit of breathing room in tight spots. If you toss it out by a downed ally, it'll give you enough time to safely revive them. It's also useful if you're surrounded at low health and just need a few more seconds to clear out nearby enemies.

Yes, that was the shield, you can upgrade either the size or the duration increase drastically, enough for reviving someone and clearing the area around it. It is probably the best support tool in the entire game. The zipline is slow to set up but can be used multiple times by multiple people at the same time, and it is rather slow, but if set up at an angle you can slide down much faster if you press W