There are 6 golden bloons per game, with the first one appearing randomly somewhere in Rounds 21 to 30.
A minimum of 9 games is required to get this achievement.
Any Golden Bloon pops count immediately towards the achievement, even if the end result of the save is a loss. The main point of surviving longer is to catch more Golden Bloons per save.
Refrain from exiting to the main menu, as that can cause Golden Bloons to no longer spawn after reloading a save.
General Tips
The easiest way to farm for this achievement is by playing on Beginner maps, although getting progress on harder maps is still possible.
Flash Bomb is the only practical solution to pop Golden Bloons, due to its reliable ability to stall the Golden Bloon until it can get popped. Buying Ninja Discipline helps make it more consistent.
The fact that Golden Bloons gain the Lead property on Rounds 41+ helps ensure that only the Flash Bomb projectiles hit the Golden Bloon, and thus stun it. Avoiding purchasing Acidic Mixture Dip can help the consistency of this strategy by avoiding the possibility of triggering the Golden Bloon's invulnerability.
Simply having more Flash Bomb ninjas is much cheaper than trying to buff a single Flash Bomb with Shinobi Tactics.
A 3-0-0 Druid is a stalling tower, but it's unreliable.
For hero choice, go for a hero who will not have potential to trigger the Golden Bloon's invincibility.
Benjamin is a good choice as he is purely a support hero that does not attack. His income generation is generally excellent for any strategy.
Psi is another good choice for this achievement, as they can never target Golden Bloons outside of their no-damage Psionic Blast ability.
Conversely, Obyn Greenfoot and Gwendolin are poor choices for this achievement, due to them being able to hit the Golden Bloon (if decamo'd) and triggering its invincibility, but dealing low damage.
Bloon Science·Gimme that!·Extortionist·Sweet Revenge·Best Served Cold·Guardian·Epic Defender·Defender·You Shall Not Pass·Attack Run·Dark Dirigible Titan
City Honor
Red City Honor·Blue City Honor·Green City Honor·Yellow City Honor·Pink City Honor·Black City Honor·White City Honor·Zebra City Honor·Rainbow City Honor·Ceramic City Honor·MOAB City Honor·BFB City Honor·ZOMG City Honor
Heating up·Afraid of no ghost·Broken Bones·Drill Bits·Top of the heap·That's a wrap·Breaking out·Fires of Mount Bloon
Blops
Blop Around the Clock·Blop Til You Drop
Medals
Poppin' and Bloppin'·Bronze Master·Silver Master·Gold Master·Diamond Master
Powerset Badges
Mixing It Up·The Art of Dart·Rang-ler·Rang-ler·Ready to Explode·Believe In Magic·Watch Me Whip·Bullet Hail·Dangerous Downpour
Epic Powers
Epic Avenger·I Feel Epic·There Can Be Only One (BSM2 Mobile)
Crate Powerups
Full Power·Power Couple·The Power of Money·Powering Up
Research & Upgrades
Power Core·Symmetry·Jack of all Trades·Reach for the Stars·R and D·Tagging Along
Hidden Achievements
Allies Assemble·Big Spender·Breaking Wind·Can't Hide From Me·Getting Crowded·Heavy Lifting·I'm So Dizzy·Look! No Hands!·Monkey Mastery (BSM2 Mobile)·Organized Chaos·Precision·Rise and Shine·Sink the Sub·Skull-diggery