[RPG] How to keep the Tarrasque “dead”

First off, I'll quote some general advice about converting settings, which is in the GM section of Savage Worlds Deluxe:

Like an original setting, identify the themes of the other game and try to adapt with a very few key world rules. Literal translations of game mechanics from other systems usually just result in cumbersome sub-systems that don't add one minute of fun to the Savage version.

I think that this advice also applies to converting monsters and other elements from video games. Fortunately, you seem to have the right mindset by considering their place within the setting and describing them as "stronger" or "weaker" rather than pulling out the mechanical numbers from the game.

As you noted, a d6 is usually the baseline for an "average" person in the setting (with "average" generally referring to an average trained person. An average soldier in the military would have d6 Shooting, an average writer who spends his days in a coffee shop will not). Everything else should be in relation to that.

If a Necromorph is noticeably stronger than an average human in the setting, give them a d8 Strength, which will put it on the level with someone who has gone above and beyond average strength training. Giving them a d10 Strength would make it only an equal match for those who are even more dedicated to it and so forth. Put another way, the higher their Strength, the more people they will be overpowering by a greater degree, and the fewer the characters who will be equally matched against it.

You specifically mentioned Smarts as a point of contention. One thing you can do is add the animal intelligence modifier, written as "Smarts d6 (A)". From the book:

Note that for some creatures, Smarts is listed relative to the animal world, and is thus followed by an (A) to remind you that this is animal intelligence, not people intelligence, so don't expect a dolphin to drive off in your tank just because it's a relatively smart animal.

It sounds to me like this might be appropriate for the Necromorphs, since you wrote that they aren't intelligent, but can use hunting tactics to great effect (sounds like they're using the "Smarts trick" combat maneuver). So they might have Smarts d6 (A) or even Smarts d8 (A) if they're very clever. For reference, enemies in the SWD bestiary like a Wolf, Giant Worm, and Lion have Smarts d6 (A). By the way, Attributes cannot go below d4, so you can't have Smarts d4-2 like you suggested.

Hindrances and Edges are generally for intelligent beings rather than monsters. There are some exceptions, such as monsters in the SWD bestiary getting things like Frenzy, but generally these are few and far between and almost always combat related (it doesn't make sense for a zombie to have the Arrogant Hindrance, for instance). Also note that if they do get them, they are usually listed under the "Special Abilities" section of their statblock, rather than an "Edges" or "Hindrances" section. This is more for concision in the statblock than anything else.

Finally, don't forget Special Abilities. Immediately before the bestiary in SWD is a list of monstrous abilities that are generally inappropriate for players, but are important to many monster concepts, such as ethereal and infravision. If you need something that isn't covered by anything else, this is usually where you put it (for instance, the wolf's "go for the throat" ability and the deal about a vampire needing to be invited into a house are new special abilities). However, less is more and you should avoid creating new special abilities unless it is absolutely essential to the concept.

If you'd like to see an example of converting video game enemies into Savage Worlds, check out my Elder Scrolls conversion, complete with a bestiary statting enemies from the games.

How to capture the Tarrasque for transportation and later use

Firstly, some base constraints. The Tarrasque (henceforth referred to as "Big T") can't be affected by any spell that requires a ranged attack roll or has a line area of effect. Also, I'm assuming that a giant Tarrasque statue isn't portable enough for your purposes. Finally, anything that involves a saving throw will have to be usable at least 4 times, since Big T can choose to succeed the first 3 saving throws with Legendary Resistance.

So, in order from simplest to most complicated:

1. The Imprisonment spell. The Minimus Containment option will do nicely for your purposes. The only downside is that it's a 9th-level spell which takes a minute to cast, and, as mentioned, you'll have to hit Big T with it at least 4 times. Even then, it's really good at making saving throws.
2. The True Polymorph spell. Pretty much the same as Imprisonment, you turn Big T into a portable object or a small creature that you can carry around. If you want to unleash it more than an hour after capture, you'll need to use Dispel Magic.
3. The Polymorph spell. This one only works for up to an hour, but as a 4th-level spell it's a lot easier to keep using it until Big T fails a saving throw. Plus if your DM is nice, you can use Conjure Woodland Beings to get 8 pixies, then have them cast Polymorph for you - 8 spells for the price of one!
4. A Mirror of Life Trapping. You need to have one, and you need to trick Big T into looking at it, and Big T needs to fail his saving throw. But other than that, it works perfectly for you.
5. An Iron Flask. This one only works if Big T is on a plane that isn't the Material Plane, and he still needs to fail a saving throw. And, you need to have one. But other than that, it works really well - it even forces him to obey your commands when you unleash him! If this is enough of a draw for you, consider using Plane Shift to get Big T off the Material Plane. It's better than it sounds, because he'll probably use up all his Legendary Resistances foiling your Plane Shift, so he'll be relatively vulnerable for the Iron Flask.
6. The Wish spell. Wish can, theoretically, do anything. It all depends on how good you are at wording the wish and how generous (or not) your DM is at interpreting it. I put this one last because it's by far the most risky - being killed by Big T could be the least of your worries compared to a wish that goes horribly wrong.