does the arrow have to hit a "living" npc to get "xp" or can i just shoot at a wall / target like ise soem npc's doing every now and then ?

currently got marksman at 47/50 for zooming and its getting harder and harder to find things to shoot its like the game has stoped spawning things to kill :/

You have to hit a living being. It doesn't have to be a NPC. Uh..... there are plenty of things to shoot at. As a marksman, I usually hit 100 skill in marksman at the very latest by level 19ish.

I don't know about the trainers, but I can give you two options to level marksman, but it requires at least 60 acrobatics:

1.) Find a troll or a dread zombie in an ayleid ruin, then jump on a ledge. They regenerate HP, and they can hit you, which makes them free targets.

2.) In the arena, when you're fighting a warrior, not a spellcaster or archer, jump on the ledge on the west. They can't hit you, turn the difficulty slider to the right and level to your heart's content.

i have 71 acrobatics im currently level 9 and i dont plan to do pretty much any quest till im at least level 20 (so that the loot will be decent and the enemies challenging + maybe teh arena will feel more like an arena and not some epic slaughter house)

Archery Training (Marksman):
Bruma at Hammer & Axe - Edla Dark Hart
Anvil - Pinarus Inventius

Reman Broder in Skingrad. If you can't find him at home, try Surillie Brother's vineyard right outside town.