head to head
Neither is simply harder - they are hard in different places. Ni no Kuni: Wrath of the White Witch demands more variance; Portal 2 demands more optimization.

Optimization · Pure Optimization
A charming JRPG carried by familiar evolution and party-build routing.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Ni no Kuni: Wrath of the White Witch leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans a little harder on execution, though the gap is small.
Ni no Kuni: Wrath of the White Witch
Hybrid action-turn combat, low input demand
Portal 2
Portal placement requires some precision but timing is lenient.
Ni no Kuni: Wrath of the White Witch leans a little harder on reads, though the gap is small.
Ni no Kuni: Wrath of the White Witch
Scripted enemies
Portal 2
No opponent; puzzles are static.
Ni no Kuni: Wrath of the White Witch leans clearly harder on variance.
Ni no Kuni: Wrath of the White Witch
Familiar catch and drop rolls
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans clearly harder on optimization.
Ni no Kuni: Wrath of the White Witch
Familiar evolution and party routing rewarded
Portal 2
Solving the puzzle's logic IS the entire game.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.