What are you wanting your character to do? Any race in mind? If you want to lay down the physical hurt go Ranger or Hateful Rager for awesome bonuses against evil outsiders. Or you could go paladin for holy smiting powers.
For combat maneuvers it kinda depends on what maneuvers you're wanting to do for which class is good at doing it. Generally though, full BAB with bonuses to the maneuver will be the best at it.
But without more of what you're wanting your character to do it's hard to give advice for it.
Id prefer a class that still focuses on spell casting, as well as being able to bring some pain either through an animal companion eidolon or some other means.
what do you mean by focuses on spell casting? are you wanting full spells or what?
Also I feel the difference between beastmode and not is just making bad choices. SO unless there's more of a difference you're meaning by that any class designed well can go beastmode.
And the only thing I know of for demons is smite and favored enemy. Otherwise I don't know of things that would effect them more then anything else.
I would say Druid. But the truth is i don't see how you can turn a druid into a demon slayer. I would say Magus but i don't know anything about them so i wont comment on them then,
But from what you have told me it sounds like a Paladin might be what your looking for. They get spell casting. tho its only 4th level spells but its there. If you get the mount you can bring the pain as you put it thru it. Tho i prefer the weapon bond over the animal but its your choice. Plus paladins are great for killing evil and demons. They can always kick in the beast mode when you consider weapon bond, smith, and spells and other abilatys they get. Just my 2cents
Sorceres can go Sylvan blood for an animal companion. Rangers get an animal companion that also has it's favored enemies. An inquisitor archetype in ACG gets an animal companion, Animal domain clerics get an animal companion, Hunter's from ACG get an animal companion. For the low low price of 2 feats (natures soul and animal ally) you can get an animal companion too at -3 character lv, boon for full. Barbarians can get an animal companion or a mount. Cavaliers can have an animal companion or a mount. Oracles can have a mount.
Lantern bearer rangers might be fun.
There's a ranger archetype specifically designed for demon slaying. The nice part about the ranger bonuses as opposed to the paladin Smite is that you can keep it going all day, rather than be limited to smites. They get some limited spellcasting, mostly utility spells. You can grab an animal companion, or take the party bond option and boost the damage output of other martials in your party vs demons.
I woulf defenantly look at the paladin. Heals and support via spells and auras and for when you want that single target ending attack, true strike and Radiant Charge is pretty much THEE encounter ender.
Also, take the Demon Hunter feat and I would search up auras under feats as there are some that boost your auras nicely :)
(Painful ancor possibly?)
Actually, look up all of the paladins abilities one by one. They get a lot of love in the feats section :)
A few notes. Demon hunter trait and zest for battle trait is a nice combo.
Glory of old trait +steel soul feat +divine grace =very resistant.
Paladins can choose either a mount or enchanted weapon with a good alignment.
Oath of vengance is very offensive heavy
Warrior of the holy light is nice but trades away spells.
XD
Go with the halfling sling smiter ^-^
As you said, the group isn't highly optimized and this is probably your best chance to play something like that :)
Besides, its usually the zany or unique characters that are the most fun, once you get in game for a while and the crunch wears off :)