Okay!
To relieve my DM of some pressure, I decided to take over his Sunday group so he can relax and play for once. However my knowledge of the world of DMing is highly lacking on the encounter side, I simply can't handle the PCs in my group. They're around level 9 at the moment.
First there's Saras, a Rogue (Thief) whom consistantly uses the ruling of still being hidden throughout a charge movement to effectively dash through the melee and smash the backline. At the moment she smahes them for about 30-50 damage each time.
Then there's Dr Joe, a character who's practically three healing classes in one, mainly focusing on Clericy stuff who pretty much heals for quite large amounts and has plenty of buffs that make the party rock around with about 27 or more AC. Not to mention the resists, pacifications and this one spell that sends an enemy into a different dimension until they roll a save.
Thirdly there's Xion, an Eladrin Swordmage whom frankly spams teleports, making it annoyingly hard to lock him down.
There's also two tanks in the group, a Paladin named Paige and a Fighter named Zorus. Whilst they aren't as annoying as the previously mentioned three, they do pose to hinder my encounters and make it easier for Saras and Xion to simply derp about.
Now I don't want to completely gimp them and move into a role of a player-hostile DM. Although I do want to provide challenging encounters and it seems I'm a couple levels behind in terms of their power curve.
...So what should I do?
This sounds like a 4e game. You might want to post that in the What's a DM to Do? forums or 4e General Discussion.
Basic questions, based on assumption this is 4e: Are you using post-MM3 monsters? What kinds of encounter budgets are you using? Have you double checked their builds with the folks in CharOp to verify they are legal?
Consider also using encounters with alternate goals.
Start deciding what 'challening' actually is, in numerical terms? Everyone was dropped to half health at some point? One player knocked to zero at some point?
Fair enough if you don't want think about it - the game (combat wise), as is, is essentially not fun for you. You don't have to put on a game designer hat to build onto it so as to fix that. But if the games not fun, it's unlikely there's any 'already in the game option' that'll fix this. It's either decline to DM or put on that Dev hat.
Here is somethif if you are looking to have them change strategies put a lot of line of sight blocking terrain. And monsters that can move attack move(add this ability yourself). This way the players will have to chase after them in order to et line of sight.
As for the healer cleric? Do more damage? Only way I can see to bring him low on heals.
Outside of that do a story/RP encounter.
Maybe have your fight encounters in more narrow passage ways so your rogue can't charge like that and your swordmage has nowhere to teleport to?
Or maybe mess around with the light or other vision impairing effects, like mist or smoke, and force them to roll at a disadvantage.
Consider adding debuffs to your encounters that will cause your players to take damage each turn, making things more difficult for your healer. This doesn't have to be any of the traditional choices, such as poison or infection. You could, for example, put them near the rim of an active volcano that's spewing ash or a burning building, causing them to take random, but constant, fire damage each turn. Or else you could turn his strength into a weakness; put them in the temple of a demon or (opposing god) to that your cleric worships, where the effects of his healing spells may be reduced or hindered in some other way.
But I have to ask; can you charge and remain in stealth at the same time? Or do you just mean sneak attack? If she charges while in stealth then, obviously, enemy NPCs will notice her. If not then just add things that will break stealth, trip wires, caltrops, rough terrain or even small animals that squeak when trodden on. You can have her roll on a disadvantage or make her roll a dex or perception check, whichever you think is most appropriate. 
And, as Iserith pointed out, your fight encounters don't always have to be killing groups of 4 to 8 NPCs. That one he made where your players have to prevent gnolls from reaching the other side looks quite interesting.