They can perform on higher grades too, but i would put them into tier B, while Dogface and Pazer are A+.
Her biggest problem of her and Prophets too, is that they hit like a brick, but with there slow reload there DPS is going down fast if you miss a shot or two. A short example:
7* Clyde Gold has a DPS of 23.159 with 14,457 damge per shot at a 6 shots clip. 7* Gold Artemis has a DPS of 22.377 with 39.958 damge per shot at one shot per clip.
So what happens if you hit 80% of your shots. Because of her slow reload speed, Artemis shoots 10 Arrows in 18 seconds. Hitting 80% of them cumulates to 319.664 in 18 seconds. Clyde can shoot about 5 clips in 18 seconds. With a 80% hitting rate he does 347.400. Even if you put the differnce in DPS intio account (14400 in 18 seconds) you still have a difference of ~25.000 damage in 18 seconds.
So in short, her DPS is not as good as it looks like from the raw numbers. Clydes Gold/Platinum is not even taken into account.
(I know these numbers are not 100% accurate, because of aiming, the time to empty Clydes clip etc. But I think they are good enough to ilustrate what I want point out)
So what do do with her. A good solution would be to lower the damage per shot, but shorten the reload time. In addition a passiv skill which allows to pierce through the cover would make it easier to hit and therefore to deal more damage.
Her Platinum solves the HP problem a bit, but 70k on 10* Platinum is not that much either. Clyde has about the same HP and is a glass canon. Low HP, but he has a fast and high damae output. But Artemis has no fast damage output, she needs some time to get useful, because her damage gets dangerous only after the first wave of heal is over and some heroes are already weakened and you take them out with two or three slow but big hits. The only problem is, that she is often taken out before, because of her low HP.