What are good places to test the Sunhallowed Elven Arrow?

a surface only dungeons with TONS of enemies

No such place exists in Skyrim or Solsteim. The undead are underground, that's where dead people rest: in tombs or catacombs. Even the word 'dungeon' means "underground", "subterranean" etc.

That's not to say you don't get them on the surface, you do, but it's a rare event [outside of the last location in the Undeath quest mod springs to mind, but even then there aren't many].

You may have to look further into mods. But it's a hard question to answer because it's quite out of context and lore. Skyrim does not do "tons" of enemies, it's a 'less is more' game. It's not Left4Dead.

I know there are such places because I've been fighting 5-8 enemies at once on surface.

One such place is

https://elderscrolls.fandom.com/wiki/Bard%27s_Leap_Summit

Bard Leap Summit.

It has tons of enemies on the surface. Also I think one wiki says it's the only "surface only" dungeon.

I tested the bow. The result is meh. It kills a lot but took so many arrows to kill them all. The sun is hard to see. Often behind clouds. On 11 AM to 1 PM it sort of works because the sun is on top (looks like skyrim is in equator?)

However, after that it's hard to see the sun.

I finally got bored, use my 2 hands death brand armors + swords, and mass murder them much more quickly.