The FIB Tactical team moves into the foundry. They want to get the crew bad. Arm Franklin with an assault rifle and lay into them after they're all inside. There are 2 groups of FIB agents after you. Deal with them both quickly.
When the FIB go down, Merryweather shows up to help with the party. Deal with the first group of Merryweather as Michael until Trevor starts talking. This is a good time to fight on as Trevor. Merryweather is coming in via a catwalk and Trevor has the best view of it.