head to head
Neither is simply harder - they are hard in different places. Portal 2 demands more optimization; The Elder Scrolls V: Skyrim demands more variance.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.

Optimization · Pure Optimization
An open-world RPG mostly about discovering and stacking exploitable progression systems.
Portal 2 leans Optimization (Pure Optimization); The Elder Scrolls V: Skyrim leans Optimization (Pure Optimization).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
Portal placement requires some precision but timing is lenient.
The Elder Scrolls V: Skyrim
Combat is forgiving; aim and timing rarely punish you.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Portal 2
No opponent; puzzles are static.
The Elder Scrolls V: Skyrim
Enemy AI is scripted and exploitable; not adaptive.
The Elder Scrolls V: Skyrim leans clearly harder on variance.
Portal 2
Puzzles are deterministic with no randomness.
The Elder Scrolls V: Skyrim
Loot tables and leveled lists inject mild randomness.
Portal 2 leans clearly harder on optimization.
Portal 2
Solving the puzzle's logic IS the entire game.
The Elder Scrolls V: Skyrim
Skill stacking, enchant loops, and exploits are deeply solvable.
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.