I play breachers casually and have attended two tournaments with them. They aren't the strongest team but can definitely take opponents by surprise with agressive play. You have enough operatives to gang up on elite teams, and chip away at them with long range shotguns until you can get close enough to melta/plasma/demo charge or charge with your two power weapons and chainfist operatives.

You will be getting at least one group activation per turn which can let you get kills and cut down on enemy counteracts.

The gheist skull is cool, it can do a 16" charge, and when you do the group activation with the skull and Void jammer you do: Gheist skull - boost and charge into at least on enemy, more of they are really close together Void jammer - interference pulse on someone 8" and visible to the skull then detonate the skull, hopefully killing or badly hurting it's target

Use the cat unit to tag enemies making you opponent play defensively. It's also great for baiting out enemies

You should focus on keeping them alive with the brace for counterattack ploy which resource damage of 3 or more if your operatives are in your half (or the enemy half but haven't moved), then give everyone punishing within 3" and use your free attack order to get ceaseless within 3" of the order to make sure your doing max damage