Anything that can be seen can be destroyed.
Corrolary: if the baddies can't be seen, they are invulnerable.
Make the challenge finding the targets, whether from cover, concealnent, misdirection, whatever. Fits the Fetchling theme as well.
The first thing I would suggest is don't invalidate their choices.
You are correct in that if they have a chance to scout things out and come up with a plan they should be able to win fairly handily, and the classes they have should mean they have a pretty good chance of successfully figuring out what is going down ahead of time. If they accomplish this, then letting them win easy is perfectly fine, the real challenge was in getting the intel and prepping properly, not the fight itself.
At the same time, as you note if they fail to be prepared they could get in trouble pretty quick. Don't 'save' them from this, that is risk they have chosen to take, and makes up for easy fights in terms of challenge. If their isn't a chance that if they don't prepare they might get really beat up, then it takes away the use of preparing. (I wouldn't worry about the healing, both the alchemist and the bard should be able to use wands for out-of-combat CLW, although if they don't prepare properly condition removal could become an issue for them.)
In general, even though it says 4 and 5 person parties are the same for CR calculations, I personally jump up the challenge by 25% for a 5 person party. I don't have any particular encounters, but generally more makes a more interesting fight than less. I'd probably try to avoid any encounters with less then 3 bad guys and logistically more then 10 becomes problematic. Usually I like to make the encounters as varied as the story/setting/area allows. For example, some PC-type bad guys, some monsters of various types. I try not to build encounters with any particular PC in mind, but if you go with variety it tends to make the abilities of different PCs shine in different situations.
If after a while you feel the the fights are easier than you expect (adjusting for it SHOULD be easier when they can prepare and it SHOULD be harder when they can't just based on this party style) then throwing the simple advanced template on pretty much everything can help. In my experience even moderately well optimized PCs need that little extra bit of disadvantage to get the proper level of challenge vs. the published challenge ratings.
Give them missions where they are the right people for the job, and failure means something other than their deaths. Rescue missions. Assassinations where no one but the target may be killed. Stealth missions where no one can know anyone was there.
This adds extra difficulty to combats (must protect the rescued person) without putting them at greater risk of their own lives.
Don't have all the enemies arrive at once. Have reinforcements arrive after the alarm is raised, if it is. Maybe some of the enemies hang back, letting others do the dying. This also lets you adjust the danger of the opposition on the fly where necessary.
Use enemies that force them to "think differently".
Flight monsters, invisible, uncorporeal, undead, magical, iwth resistance and immunities, teleport etc..
A slew of goblins
With a single Nilbog with barbarian class levels.
Environmental hazards could be a good asset here, making a trivial combat become deadly.
The combat is in a burning/collapsing building.
The combat is on a steeply-slanted roof with a group of dogs (re-skinned from wolves for their trip attack) waiting for anyone to fall off, and keep them from climbing back up without problems.
The combat is at night in a heavy rain storm. Everyone has total concealment more than 10' away.
The combat takes place near a tear in space, resulting in unstable magic (roll on the wild magic table any time someone casts a spell).
The combat takes place in the sewers. tactical cover is important. Also, rat swarms aren't all that uncommon...
Some wild animals broke out of their cages at the start of combat. They're hungry, confused, and disoriented. Both the PC's and the enemies are targeted by the animals.
A sneaky unseen Druid constantly harrows the PC's with Entangle spells.
The enemies know the group is coming. They set up their bear traps and take their potshots from behind cover and out of reach.
Remember you can always balance combat mid-encounter as long as you are not playing an AP or PFS. Have a few different variants of the combat around, if they do better than expected throw the harder version at them. If they are having a hard time, throw the easier encounter at them.If you cannot adjust the encounter on the fly, then use the initial fight to gauge their effectiveness and then adjust all other encounters up or down as they progress.
Players should be rewarded for gathering good intel, that intel should also not always be correct however. Sometimes guards patrol, shifts change, or they miss something important. Knowing your player's perception scores and rolling them secretly while they are scouting lets you know what to describe, and what not to describe. Without giving away that there is something to see.
Also, try to tailor encounters around their party. They are a very bursty party and their only frontline will be the Eidolon. If you want to challenge them, toss them ambushes when they rest but have not yet regained their resources. In a normal encounter have a few foes in the front draw them in and then ambush their backline. Or have the villain flood them with weaker minions to expend their resources before appearing himself.
I do rather like the "lots of zombies attack you on a hill" scenario where the necromancer only pops out after the players have fought his 50HD of zombies and skeletons.