How Arena Trials Play

An arena is one open room covering the whole battleground, with 12 or 13 spawn points round it.
Wave 1 arrives 5 seconds after the party does. Each later wave comes 4 seconds after the previous one is fully cleared.
All 18 arenas have exactly 4 waves, with 3, 4, 6 and 8 goons.
When a wave spawns, each goon appears at a spawn point (taken in turn from a random start, shifted by up to three-quarters of a block) in a puff of grey smoke, already facing and targeting the nearest party member. A red action-bar message says "Wave <n> of <total>" and a raid horn sounds.
When the last goon of a wave dies, a green message says "Wave cleared. The next approaches..."
After the fourth wave you see "The trial's master stirs...", and 4 seconds later the boss appears.