Quincy and Gwendolin can use their abilities to damage their own Golden Bloons and get extra Gold.
Mortars have no control over attacking a bloon at the end of a turn. If only Golden Bloons are on screen, you can force the opponent to give you more Gold.
Against Quincy, his Perceptive Shot passive will activate the on damaged effect.
The card would essentially cost 1 Gold when played this way, and you get the same Bloontonium.
It might be helpful to tank Golden Bloons, or delay attacking them, but only if you know the opponent won't be able to afford anything that would save themselves.
Attacks that deal at least the Gold Bloon's health will ensure that the sender will only get 1 Gold, no more.
For wholescreen options, Firestorm one-shots Golden Bloons if not playing on Dark Castle.
Sniper Monkey one-shots Golden Bloons if they're not health-buffed by cards.