

So it depends on the spell, but I think you are talking about summon nature’s ally. That allows you to give instructions to creatures who can understand, and they will fight to the best of their ability, but as a DM I wouldn’t interpret the spell as written to include suicide.
But even then, a good DM doesn’t put a tarrasque into play and have it sit there and die. Once it realizes it is getting damaged and can’t retaliate, it can burrow from we whence it came, etc.
So I think most of the strategies involve weak roleplay from the DM, munchkin builds, liberties with the rules, or both.
Even then, actually killing the tarrasque requires a wish spell, which is not something that a 9th level druid can do.
I remember buying mistmare on cd back in 2003. That thing was a broken mess of a game that crashed constantly, and no returns once you open the seal. Kids these days don’t know what a 1/10 game really is, lol. That game was so bad most of the (short) Wikipedia page on it is about it’s low scores, including a 0/10.