1. Go to the 4th floor of Eos tower and enter the hidden street map behind the dark block golem head surrounded by screws

2. In this hidden map, Rombot spawns in 2 locations, on the platforms 2nd to the top and 2nd to the bottom

3. Rombot has a spawn timer of 15-20 mins in both locations. If it's busy and people are cycling through ccs, camp one of these locations and clear the platform for however long it takes for Rombot to spawn, you don't need to clear the rest of the map. I recommend placing summons if you have them to immediately kill it when it appears. Otherwise just cc, kill a few mobs to force a spawn cycle, and check these locations.

EDIT: Removed drop gear bit since it seems 66% won't guarantee it, but wear as much as you can