How do I get legendaries at my level?

You need to beat him at your level in order to get a higher level drop. On the first playthrough the areas don't scale So he'll always be level 1 and drop level 1 gear.

You need true vault-hunter mode (which will cause all their areas to re-scale starting from 30) or ultimate vault-hunter mode (where areas scale to your current level). To unlock those you have to beat the game, but once you have you can go back and forth between replay modes and you keep all your gear.

You should always farm at the highest level that you've unlocked if what you want is gear.

Raid bosses are for once you've beaten level 50. They drop the best loot, but are really intended to be a tougher challenge for UVHM and levels 50+ They are quests given out once you've cleared all the story quests in an area.