Hi I have just gotten a rainbow wing but I want to know which level to..

I think you should be atleast a level 45,it's not because your weak iy's just like me you have starters that are at a disadvantage against Ho-oh.Maybe if you save your water moves and teach Charzard Earthquake and you probably beat Ho-oh.
PS:Ithink you have to get SKY BLUE PLAINS to unlock JOYOUS TOWER (your level is set to 1)Also Flig is super effective on ground.

It's very likely that Ho-Oh will join your team, but since he's at level 50 when you'll be trying to recruit him, you MUST have a Pokemon higher then level 50 as well as meet the other requirements for recruitment(party size less then 6 stars-he's a 4 star size so you can only bring 2 of your 3 listed one star sized Pokemon or dismiss one before reaching the top; leader being in adjacent tile and dealing the final blow) and though you cannot get him first time through, I had no problem the 2nd time and of course you need his friend area, too, before he'll join (Rainbow Peak-get from Wigglytuff).
Easiest thing, go look in answers above for Joy Seed WM Codes and just level one of them up to 51 before even trying to recruit him. He's actually not that hard to knock out, use items like iron thorns and silver spikes(or let yout team use them), and arrange your Pokemon so they can help you out and limit their attacks to the most effective ones only.
Having Sky Blue Plains friend area will get Joyous Tower on you dungeon list.