There are two dumps of Mercs that I know about: Mercs (JUE) [!].zip and Mercs (JUE) [h1].zip. The latter has "MERCS" set as it's Domestic Name, while the first has something else, maybe "Senjou no Ookami 2" in Japanese. There are no other differences between the two ROMs.

If you want to play the game as "Mercs", then set the emulator to US or EU mode, if you want to play "Senjou no Ookami 2", then set it to JPN mode. Note that the only difference will be the Japanese text in some places like the title screen and the shelters in Original Mode.

Emulators:

- Kega Fusion: I recommend using Kega Fusion. Runs perfectly in every way. Use SuperHQ Sound.
- Gens: Works fine. Set the sound quality to 44100 Hz, and use the YM2612 High Quality Mode (Shift + F12) to get the best results.
- DGen: The game runs fine, but the Frame Rate is very low, and and the graphics are glitched up in several places. And the sound is strange too.
- Kgen: The game runs fine, but there are lots of glitches, and the sound is bad.
- Genecyst: Haven't tried, but it should be working like Kgen.