Solosis - mid tier or limbo between middle and low tier
Availability: as soon as you get to Nimbasa (exclusive to White 2)
Stats: Reuniclus is one of the most powerful non-legendary Pokemon available in your walkthrough; its only drawback stats-wise is very low speed. However, Solosis and Duosion are somewhat fragile and slow but possess a very high SpA stat.
Typing: Not great, but not too awful either.
Movepool: Its level-up movepool is awful. In fact, it's recommended you wait until you get to Strange House to capture one because you'll only have a crappy Hidden Power to work with for the first few levels and Solosis' relative fragility and low speed do it no favors. Keep Hidden Power until you get decent TMs because, depending on what it is, it will be the only thing it has to prevent being completely walled by Dark and/or Steel types. Reuniclus is renowned for its excellent movepool in competitive play but you will not have access to these resources during your playthrough. Recover is nice to have, though.
Major Battles: Meh. The abundance of Dark and Steel types do it no favors. As Reuniclus, it is bulky enough to not be useless against the Ghost and Dark E4 members, but it's not the most reliable thing to have.
Additional Comments: As good as Reuniclus is, the pain of getting there isn't worth it. I would recommend you skip it, especially if you are doing a no-trades walkthrough.