Ants are numerous and they are powerful when together. Unfortunately, it would take thousands if not millions of ants to overwhelm a Kaiju as large as Biollante. Biollante can just use her acid blast to take out the Ant and win the round. It must have been difficult to see her take a loss against Havok in the last match, but it would appear that Biollante has finally regained her momentum. Biollante wins.
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What if Biollante went up against an army of SUV sized bullet ants. They can lift 20 to 50 times they’re weight so an SUV sized one can probably lift a whole train with no effort. Their sting is said to be as painful as a gunshot wound hence the name. So an SUV sized bullet bite would be about as painful (and dangerous) as a rocket propelled grenade. Surely the ants can win then.
Then it would be tricky, but Biollante can just keep on shooting acid to keep them at bay. Long range abilities sometimes do end up trumping numbers. Of course, it would definitely be a very close fight