If an effect allows you to cast a spell “as though” it has flash, Slitherwisp’s last ability won’t trigger.

In a Two-Headed Giant game, Slitherwisp’s last ability causes the opposing team to lose 2 life and you to draw one card.

An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Players can cast spells and activate abilities after the triggered ability resolves but before the spell that caused it to trigger does.