The druid class

[Shadowbane13]

Hello, my friends and I have started a new campaign and A friend is being a Druid and would like some assistance in creating a powerful caster. Thank you

[GilesTheCleric]

Eggynack created an excellent handbook (http://www.giantitp.com/forums/showthread.php?439991-Being-Everything-Eggynack-s-Comprehensive-Druid-Handbook) on the subject. However, it's a lot of reading. My one-sentence advice is "do as you please". Druids are very capable no matter what you do with them, so your friend is free to go in any direction. All they need to do to contribute to the party is to cast an entangle every now and then, or to turn into a bear or eagle or something.

[Bronk]

I agree. For spellcasting only? Just remind them to check through all of the allowed books allowed by the DM for spells (since they're all on his/her spell list), point out that the Natural Spell feat will allow casting while wild shaped (they can cast while safe in the sky, or just without changing back while wild shaped), and remind them that their animal companion gets 'share spells', so any spell that can be cast on themselves will also be cast on their companion (which includes some attack spells).

[Chilxius]

I see druids as having three main skills: casting, wild shape, and animal companion. They do fine using all three, but if you want to focus on just one there are feats/prestige classes to make a good specialist.

For Wild Shape:
Master of Many Forms (CAdv 58) - A good utility shapeshifter. Yuan-Ti Abomination is a great one for mid levels.
Nature's Warrior (CW 63) - Never tried this one, but looks like so much fun. Good BAB.
Warshaper (CW 89)

Animal Companion:
Beastmaster (CAdv 26) - You probably only want 1 or 4 levels of this one.

You probably want to do a pure druid for effective casting.

Feats to keep in mind for multi-classing are Practiced Spellcaster(CArc 82) and Natural Bond(CAdv 111). The bonus from Natural Bond and the first level of Beastmaster stack in whatever order is most beneficial.