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Keeping it down is easy

Dealing continual damage will keep the Big T down for the count as long as you want to keep it down. Drowning him or placing him in an environment that causes continual damage that he can't resist (or something more exotic - Positive Energy Plane?) can do the same job. Big T can't burrow, so you can drop him in a deep hole (dug with magic, probably) and then cave it in on him.

Making Him Dead, Though...

Essentially you'd need a way to strop him of the regeneration trait entirely. 3.5 had a psuedo-poison (Trollbane) that would suppress Big T's regeneration, but that content isn't available in Pathfinder and furthermore their version of the Big T ignores it by virtue of your quoted text. Without a way to strip the ability entirely the best you can really do is kill him, summon a small army of flying things that don't need to breathe, and ask them to drop him off in the vacuum of space.

And Requested By Comment

Further clarification on the Positive Energy Plane was requested, so:

The Tarrasque is not immune to being blown up by the Positive Energy Plane, though it is immune to dying thereby. Dropping him into the Plane is mostly to get rid of him from your dimension; lacking a native way out, he heals, overheals, and then 'dies', returning to 1 hit point to restart the cycle. The Negative Energy Plane isn't as useful because the Tarrasque regenerates the damage it's dealt every round, and even if it comes into contact with some aspect capable of killing it instantly Big T's Fort save is more than sufficient to protect it.

Keeping it down is easy

Dealing continual damage will keep the Big T down for the count as long as you want to keep it down. Drowning him or placing him in an environment that causes continual damage that he can't resist (or something more exotic - Positive Energy Plane?) can do the same job. Big T can't burrow, so you can drop him in a deep hole (dug with magic, probably) and then cave it in on him.

Making Him Dead, Though...

Essentially you'd need a way to strop him of the regeneration trait entirely. 3.5 had a psuedo-poison (Trollbane) that would suppress Big T's regeneration, but that content isn't available in Pathfinder and furthermore their version of the Big T ignores it by virtue of your quoted text. Without a way to strip the ability entirely the best you can really do is kill him, summon a small army of flying things that don't need to breathe, and ask them to drop him off in the vacuum of space.

Keeping it down is easy

Dealing continual damage will keep the Big T down for the count as long as you want to keep it down. Drowning him or placing him in an environment that causes continual damage that he can't resist (or something more exotic - Positive Energy Plane?) can do the same job. Big T can't burrow, so you can drop him in a deep hole (dug with magic, probably) and then cave it in on him.

Making Him Dead, Though...

Essentially you'd need a way to strop him of the regeneration trait entirely. 3.5 had a psuedo-poison (Trollbane) that would suppress Big T's regeneration, but that content isn't available in Pathfinder and furthermore their version of the Big T ignores it by virtue of your quoted text. Without a way to strip the ability entirely the best you can really do is kill him, summon a small army of flying things that don't need to breathe, and ask them to drop him off in the vacuum of space.

And Requested By Comment

Further clarification on the Positive Energy Plane was requested, so:

The Tarrasque is not immune to being blown up by the Positive Energy Plane, though it is immune to dying thereby. Dropping him into the Plane is mostly to get rid of him from your dimension; lacking a native way out, he heals, overheals, and then 'dies', returning to 1 hit point to restart the cycle. The Negative Energy Plane isn't as useful because the Tarrasque regenerates the damage it's dealt every round, and even if it comes into contact with some aspect capable of killing it instantly Big T's Fort save is more than sufficient to protect it.

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Keeping it down is easy

Dealing continual damage will keep the Big T down for the count as long as you want to keep it down. Drowning him or placing him in an environment that causes continual damage that he can't resist (or something more exotic - Positive Energy Plane?) can do the same job. Big T can't burrow, so you can drop him in a deep hole (dug with magic, probably) and then cave it in on him.

Making Him Dead, Though...

Essentially you'd need a way to strop him of the regeneration trait entirely. 3.5 had a psuedo-poison (Trollbane) that would suppress Big T's regeneration, but that content isn't available in Pathfinder and furthermore their version of the Big T ignores it by virtue of your quoted text. Without a way to strip the ability entirely the best you can really do is kill him, summon a small army of flying things that don't need to breathe, and ask them to drop him off in the vacuum of space.