Anyway, you are right, you can of course roll an evil character, but if you don't have one, that Allip-idea might not be so easy.
Also, see this part from the D&D wiki:
Should work against an Allip, I think, unless incorporeal creatures are again an exception.The tarrasque’s natural weapons are treated as epic weapons for the purpose of overcoming damage reduction.
Incorporeal doesn't grant damage reduction. They just can't be harmed unless you are using magic. The tarrasque's natural weapons are treated as epic weapons for the purpose of overcoming damage reduction, but are not epic weapons. They are just mundane claws and teeth, although very big.
The Drain/Damage distinction I'm not sure of. As I said, I'm no expert on D&D rules. Discussion on the internet seem a bit confusing about that.
As I said, you will probably be at the mercy of the DM.
Also, while the allip example might be subject to debate whether the rules mean one thing or another, we were talking about a level 5 cleric that wouldn't even get XP from killing the tarrasque. If the DM is throwing an epic monster at a low level party (even if it's an overrated), he probably doesn't really want to play. By level 15 killing a tarrasque is but a chore for most reasonably prepared parties. The fact that it's but a meaty damage sponge, unable to cast spells, use interesting spell-like abilities, or even fly, make the tarrasque a minor annoyance at best.