If you're tackling move-viewing tasks, Obsidian fields is your best friend for this.
Once there, begin fighting as many low level Pokemon as you can. (Obsidian fields is actually great for this as it also has plenty of berries and herbs to find to restore your Pokemon.) The low level Pokemon will typically faint after one move, but there are plenty of them. Quantity over quality is key here.
If you really only want to do one battle, there is an alpha blissey there. Blissey is a super tank-y Pokemon, though the alpha blissey is a high level, so the many low level Pokemon approach is probably better for this with minimal fainting.
Source: I complete all the move viewing research tasks this way.
