Berta, Wise Extrapolator

Berta's second ability isn't a mana ability. It uses the stack and can be responded to.

When comparing the mana spent to the creature's stats as the increment ability resolves, it's possible that the stat that is less than the mana spent changes from power to toughness or vice versa. For example, say you control a 1/3 creature with increment and you pay two mana to cast a spell. Since the amount of mana you spent is greater than the creature's power, increment will trigger. While the increment ability is on the stack, an effect gives the creature with increment +2/-2, making it a 3/1. When the increment ability resolves, the creature will still get a +1/+1 counter since the amount of mana you spent is greater than that creature's toughness.

The increment ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.