head to head
Neither is simply harder - they are hard in different places. Clash of Clans demands more reads; Portal 2 demands more execution.

Optimization · Pure Optimization
Base-building strategy game about optimizing upgrade economy and raid compositions.

Optimization · Pure Optimization
A first-person puzzle game where seeing the solution is the whole skill.
Clash of Clans leans Optimization (Pure Optimization); Portal 2 leans Optimization (Pure Optimization).
Portal 2 leans clearly harder on execution.
Clash of Clans
Troop drop placement is forgiving
Portal 2
Portal placement requires some precision but timing is lenient.
Clash of Clans leans clearly harder on reads.
Clash of Clans
Base layouts anticipate human attackers' habits
Portal 2
No opponent; puzzles are static.
Clash of Clans leans clearly harder on variance.
Clash of Clans
Matchmaking and unseen traps add uncertainty
Portal 2
Puzzles are deterministic with no randomness.
Portal 2 leans a little harder on optimization, though the gap is small.
Clash of Clans
Army comps, upgrade paths, and attack routing
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.