It doesn't matter if the first spell you cast that turn resolved, didn't resolve, was countered, or is still on the stack
The best part is that the game offers two free booster packs daily, so everyone has an opportunity to add to their collection
If one of the two creatures leaves the battlefield before Secret Tunnel's ability resolves, the other still can't be blocked this turn as long as it has a creature type that the other card had as it left the battlefield
The best among them are 3-color legends like Fire Lord Azula and Sokka, Tenacious Tactician that are both powerful and flexible, so you can build them however you like
Even heavily damaged copies of this printing are now listed for $25
Invasion of the Giants 1 Printing and its Price Track Invasion of the Giants Prices Free collection tools for Magic: the Gathering