Test: The test is that 1st target and a single instance of a enemy type (eg. 1 atlas which is the 1st target + 1 nemesis) would be isolated and missiled. This would check whether the enemy would respawn as the 2nd target. This also shows that you don't need to kill a lot of stuff together, just the 1st target and a valid enemy.
Shortform: In general, the 2nd target will never come from the "unlimited" category in the wave composition. The exception is when the target is limited in the number of times they can appear in a wave such as a praetorian. If you kill the non-target praetorian in wave 10 repeatedly for 5 times, when you kill the target praetorian, the next target will be from the "unlimited" category.
Wierd Exception: Reapers wave 6. The brute cannot be changed to a marauder or ravager. Missile all 8 enemies on the map at the same time and you'll still get a brute as the 2nd target. Because of this exception, there is no general rule (wave budget cost, enemy size, etc) that can be used to explain why wave switching exists
Disclaimer: Note that this is on PC. No idea if there is any difference with consoles. Also i'm lazy, this also only tests for the 1st target and only for gold.
*****************Gold Cerberus***************
Wave 3
Initial Boss: Atlas
Unlimited: AssaultTrooper
Confirmed switch: Nemesis, Phoenix
Note: Kill all dragoons = 4th target is an assault trooper
Wave 6
Initial Boss: Phantom
Unlimited: Nemesis
Confirmed switch: Engineer, Phoenix
Wave 10
Initial Boss: Atlas
Unlimited: Engineer
Confirmed switch: Phantom
*****************Gold Geth*********************
Wave 3
Initial Boss: Geth Prime
Unlimited: GethTrooper
Confirmed switch: GethBomber, GethPyro
Wave 6
Initial Boss: Geth Prime
Unlimited: GethHunter
Confirmed switch: GethRocketTrooper, GethPyro
Wave 10
Initial Boss: Geth Prime
Unlimited: GethRocketTrooper
Confirmed switch: GethPyro, GethHunter
***********************Gold Reapers***************************
Wave 3
Initial Boss: Banshee
Unlimited: Cannibal
Confirmed switch: Marauder, Brute
Wave 6
Initial Boss: Brute
Unlimited: Marauder
Confirmed switch: None
Confirmed switch fail: Ravager
Wave 10
Initial Boss: Banshee
Unlimited: Marauder
Confirmed switch: Ravager, Brute
***********************Gold Collectors***************************
Wave 3
Initial Boss: Praetorian, MaxPerWave=3
Unlimited: CollectorTrooper
Confirmed switch: Abomination, CollectorCaptain
Note: Killing all praetorians will spawn a collector trooper as the 4th target
Wave 6
Initial Boss: Praetorian, MaxPerWave=6
Unlimited: CollectorTrooper
Confirmed switch: CollectorCaptain, Scion
Note: Killing 5 praetorians then the 1st target will spawn a collector trooper as the 2nd target
Wave 10
Initial Boss: Praetorian, MaxPerWave=6
Unlimited: Abomination
Confirmed switch: CollectorCaptain, Scion
Platinum Target Changing
For platinum, the mixing of factions seem to kill any chance of target switching.
Only 2 waves out of all the possible 12 objective waves allow switching though that makes it easier to remember -> Reapers 6 and Collectors 3. The value of target switching in both of these waves isn't useful much (initial target of atlas and brute respectively and supporting troop types are not really dangerous)
From the results, assuming that there was no hardcoding to fix the target to only X when they added platinum, there's no fixed spawn order sequence nor does size matter. In reapers 10, you cannot switch a banshee to a prime and in geth 10, you cannot switch a prime to a banshee. If there was a spawn order to it or if size/animation complexity mattered, one of those waves would be switchable.
Conclusion: For platinum, you're stuck with shooting the targets down or expending missiles to keep the objective short.
*****************Plat Cerberus (no switchable waves)***************
Wave 3
Initial Boss: Atlas
Unlimited: Phantom
Confirmed switch: None
Confirmed switch fail: Brute, Ravager
Wave 6
Initial Boss: Atlas
Unlimited: Guardian
Confirmed switch: None
Confirmed switch fail: GethBomber, GethHunter
Wave 10
Initial Boss: Atlas
Unlimited: Phantom
Confirmed switch: None
Confirmed switch fail: GethBomber, Banshee
*****************Plat Geth (no switchable waves) ***************
Wave 3
Initial Boss: GethPrime
Unlimited: GethPyro
Confirmed switch: None
Confirmed switch fail: Centurion, Engineer
Wave 6
Initial Boss: GethPrime
Unlimited: GethBomber
Confirmed switch: None
Confirmed switch fail: Brute, Ravager
Wave 10
Initial Boss: GethPrime
Unlimited: GethHunter
Confirmed switch: None
Confirmed switch fail: Phantom, Banshee
*****************Plat Reapers (switchable only on wave 6)***************
Wave 3
Initial Boss: Banshee
Unlimited: Marauder
Confirmed switch: None
Confirmed switch fail: GethHunter, GethPyro
Wave 6
Initial Boss: Atlas
Unlimited: Brute
Confirmed switch: Ravager, Guardian
Wave 10
Initial Boss: Banshee
Unlimited: Ravager
Confirmed switch: None
Confirmed switch fail: Phantom, GethPrime
*****************Plat Collectors (switchable only on wave 3)***************
Wave 3
Initial Boss: Brute
Unlimited: CollectorTrooper
Confirmed switch: Abomination
Confirmed switch fail: Ravager
Wave 6
Initial Boss: Praetorian
Unlimited: Abomination
Confirmed switch: None
Confirmed switch fail: Atlas, Engineer
Wave 10
Initial Boss: Praetorian
Unlimited: CollectorTrooper
Confirmed switch: None
Confirmed switch fail: GethPrime, Banshee
edit: corrected typing errors on collectors
Edited by Computron2000, 15 August 2013 - 04:16 PM.