Note: this answer is somewhat similar to @THiebert's, but elaborates more.
TL;DR: Diversity rocks.
The problem of the solitary Big Bad Wolf is that it will quickly get stopped in its tracks (Barbarian), the Rogue will easily slipped behind to get into flanking position (Tumble), and the Druid and Wizard will be left alone, free to use their most potent spells.
In turn, this require the Big Bad Wolf to be real tough to survive just a couple rounds, and/or to have very debilitating abilities to be able to be a credible threat (knocks, stuns, teleports, or just lots of raw damage).
It can be satisfying to take a big burly enemy out by the skin of your teeth, with half the party KO and only a few HPs remaining... but the problem is that it's really complicated to get this close to death without killing. Most notably, note that getting a PC to below 0 but above -9 requires a rather precise amount of damage...
I believe a GM should tweak its rolls to further story-telling, but here that's too much work.
Diversity is a very simple measure to get more satisfying encounters for everyone involved.
By moving away from the solitary Big Bad Wolf you get multiple benefits:
- multiple spread out foes cannot be taken out by a single PC, no matter how lucky,
- multiple foes can have various abilities: for example a group of goblins can have a shaman (druid), 2 archers and 2 melees.
By simple encounter mechanics, multiple foes must be weaker than a single foe:
- any PC has a chance to one-shot a foe from time to time,
- there are less risks of PC death since the foes strike are not as strong (avoid focusing too much),
- the encounter is less swingy as the foes' attacks are not ALL-OR-NOTHING, the more foes, the more average the damage,
Furthermore, multiple foes, especially if diverse, allow for greater tactics:
- they can spread out, forcing the party to use battle-field control,
- one or two can harry the casters, putting pressure on them (and forcing them to seek cover),
This means that the fight becomes more one of tactics than one of luck, which is more satisfying.
It also matters less in such a fight if one PC (the rogue...) is less powerful than the others; there's necessarily a role that it can play to help/relieve its allies anyway.
Once you have diversity, the second trick is waves.
There's no reason for the whole group of NPCs to appear at once (at least, not all the time).
The great thing about waves is that:
- they put time pressure on the PCs: be too slow, and you'll be overwhelmed,
- they allow you, the GM, to adapt the fight without a Deus Ex Machina: adjust the size/strength of the wave off-screen depending on how well the PCs are doing compared to how much resources you wish they expended,
- they allow you, the GM, to put pressure on the part of the battle field you wish for, as the next wave arrive where you wish it to.
No need to fudge dices, or to inexplicably "flee" or "retreat". The party will never know that you had 4 orcs scheduled but only put in 2 instead since the Rogue was already unconscious.
Now, I don't mean to go full kobolds on the PC (though Tucker's kobolds are frightening), and not all encounters should be the same, but just moving from one Big Bad Wolf to two (smaller) Big Bad Wolves that attack the party from opposite sides will give you more interesting and less swingy fights.
Finally, you may wish to help your players buff up their characters. ECL 11 and 50 hp? This character needs to buy an Amulet of Health +2 or +4. Yesterday. And look into temporary HPs: a Wand of False Life would be very helpful.
I won't expend too much as it seems to be another question, but the short of it is that the game expects players to do their homework (for better or worse) and shore up their weaknesses. You need some HPs, some ways to "get out of jail", some ways to reach flying enemies, some ways to protect yourself from mind control/negative energy, ...