This is an old question but I just wanted to answer some general points. If you are only level 5 in Pokemon Go you won't be able to defeat legendary raid bosses. Pretty much all of them require multiple people even with high level Pokemon (L35+).
So you will need to find multiple people with L30+ accounts to join you. There are apps like PokeGenie that can give you remote raiders or you can join others remotely.
For raid counters you can check out Pokebattler. However, the recommended team that shows up in Go is quite good these days, as long as you have some high level Pokemon with super-effective moves. In general defense doesn't matter too much in raids, as they are timed and you need to deal as much damage as possible quickly.