What is the best way to drop good weapons for a char you just unlocked ?
You always get one weapon for the character you're using. Anyway, just go to some higher tier missions in the original adventure map. If you can play in co-op, make one player go with a strong character while the other waits. If you can't, then you can just level up the character with rupees before sending them there. If you don't have rupees, then try your luck by playing in those stages with random leveled up characters while gathering rupees and hoping for weapon drops. Alternatively, there's always the random net Links.
Edit: Wait, I was thinking about DLC characters, if you're talking about the base game, then, you probably don't have a complete map yet, so, really the best way is just to play as them. You're guaranteed one weapon drop. For higher ranking ones, you need to fit unlock them in the Adventure map (or story mode for a few story characters, although all lvl 3 weapons are in the Adventure map only).