I just find that people usually forget that experience and tactics are not absolute, and you can't really be prepared for everything. IF it was possible then Batman would have never had his back broken by Bane who had way less experience and preparation than himself. That's the problem I have with smart solutions(or finding ways to use a mediocre power in a way that makes it superior than other well round abilities), they only work when you are in control of the situation. Which is why when a character is powerful enough to be considered op, he is op because he doesn't need to rely on his brains to do the job, he is op because even if the enemy can use his brains to read the situation, the mc will still find a way to ignore his planning by simply overpowering whatever the enemy can thow at them. Moreover if the other guy is an assassin, what it means that the guy is smart enough to strike when the enemy is least expecting. Assassins are good because they always know when to strike, they don't need to deal with a reincarnator's or an Isekai Hero op powers, they just need to kill them before they use their cheats, good assassins are effective because they always have the higher ground(they always investigate whoever they want to kill). However this also means that if someone were to expect to be assassinated, and knows the assassin is nearby, it would force the assassin into a direct fight, and when this happens, more often than not the assassin dies, no matter how skilled of an assassin he was. I don't know what really happened for real, but if the assassin didn't have the MC as his target, and if the MC found him in his state, than it doesn't matter how successful he is at assassinating Isekai'd people, I can easily see the MC one shotting the guy