I checked the math, and the sheet, and cross referenced it with the phb, and watched countless videos on stealth, surprise, sneak attack, crits, assassinate etc etc.As mentioned above, check the math and the character sheet. Try to build this Assassin yourself from scratch. If you can't do it - even knowing the desired goal - sit down away-from-table and have a talk: how did HE do it?
When I ran Arauthator in Rise of Tiamat, I amped him up as high as I could within the rules: he became an Ancient White Dragon (not Adult), with max HP &c. The Ranger still got a crit with an Arrow of Dragon Slaying and took out 1/5 of his total HP in one shot. I didn't have to act very much when I started wheezing and choking and holding my ribcage.
Assassins have to set up an ambush to get all their cool features to work right. Is that what your group is doing, or are they kicking down the monsters' front door?
If the group is making a name for themselves, have some NPC Rogue in the story try to ambush them back - because if he pulls it off, HE is the big bad toughie of the neighborhood. Roll with your PC's in-story reputation !
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