It might be a stupid question, but as yet there is a Final Word for LIMITLESS RAGE power. Many people interpreted literally.

LIMITLESS RAGE: Hulk can use Toughness. Hulk ignores all damage dealt to him unless it is dealt by an attack and the attack roll is doubles, but not a critical miss. Friendly characters cannot use Probability Control during an attack that targets Hulk. This power can't be countered or ignored.

is perhaps the only way to damage Mulk, with attacks that rolls doubles? It may be damage from Mystic TA? with Pushing Damage?, with maps as Krakoa or Hawkworld?, with Poison + Armor Piercing? or otherwise that does not involve an attack?

Unavoidable damage (like Mystics, Critical Misses, etc) will damage Hulk normally. This is because unavoidable damage can't be ignored.

If you counter Hulk's Indomitable, pushing damage will also be effective since pushing damage can only be ignored by effects that specifically say they ignore pushing damage. Limitless Rage does not specifically mention pushing damage.

Otherwise, you need to attack him, roll doubles, and deal at least 2 damage (because his Toughness will prevent 1 damage dealt from hurting him).

then, the maps do deal Unavoidable Damage as Hawkworld, Krakoa and Muspelheim, normally would do damage?

Hawkworld
Orange squares are Open Air (Low Gravity). After actions
resolve, if a character who can’t use the Flight ability occupies an
Open Air square, deal them 1 unavoidable damage, then that
character’s owner places them in their starting area. Non-held
objects in Open Air squares are removed from the map. Open Air
squares are otherwise treated as clear terrain.

Krakoa
On a result of 5, SHIFTING FOLIAGE. Printed hindering terrain
acts as blocking terrain instead until the beginning of your next
turn. Characters occupying those squares take 1 unavoidable
damage and are placed in their owner’s starting area.

Muspelheim
Orange squares are Lava Squares. At the end of each player's
turn, deal 1 unavoidable damage to each character occupying a Lava Square.

Yes you have it right.

I'd say those would work.