No surprised, you casted a verbal spell (Hunters Mark).
Weak, multiclassing and Darkness situational. Pretty bad build
Go without Multiclassing
What do you mean 'not surprised'?
He could just be selecting you as the target with his bonus action (having previously cast the spell in a prior encounter) but even if he isn't, it's a bonus action to cast.
He sneaks up to you (Stealth +13; invisibility in darkness) and then declares he casts HM as a bonus action and attacks you, triggering initiative, with those things (Hunters Mark and Attack action x 2) happening on his first turn (and you're surprised on turn 1 and cant take any actions or move or reactions till after your turn).
He's got +7 to initiative so likely goes first, meaning no reactions till after your first turn (and after he's already unloaded 6 Bow attacks).
The Sorcerers AC is likely no better than 16 (Mage armor and 16 Dex) and being unable to cast shield (due to surprise) so at +6 with tri-vantage they're all likely hitting.
Heck, even with shield, and with no tri-vantage to help out, he just drops Sharpshooter and at +11 to hit vs AC 21 he only needs 10's to hit (spamming a d8 superiority die on precision attack on any shots he misses by 5 or less).
That works out to around 5 hits on average (burning 2 superiority dice to help land a hit), for around 10d8+5d6+25 (around 90 or so points) of damage in one turn.