For starters, lets look at the starting stats: the overlord has a deck of cards that replenish at a rate of one per turn and an assortment of monsters, some of which he must use and some of which he must not use for any given quest. These monsters might replenish at a rate of 1-2 per turn. By contrast, the heroes start with two abilities (one on the character sheet and one on the card), some of which are insanely good, that they have access to on every turn and a feat, which is always insanely good, that he or she can use twice per quest. It is worth noting the significance of the fact that a hero will always have access to at least two useful abilities each turn, whereas the overlord cards are generally quite situational, are available by random chance, and can rarely be reused in a quest.
Try an OL using 2x Unholy Ritual in his deck with 9 Kobolds out. He's drawing 1 card as normal, then playing UR to draw 9 cards and choosing to keep 4 (9 Kobolds with 4 heroes), he'll see pretty much half his deck every turn. Throw in Reinforce and now it doesn't even matter if you wipe out the Kobolds, he'll Reinforce and then UR the Kobolds to very likely re-draw Reinforce (keeping enough cards in hand to allow him to draw Reinforce again). Start of most Encounter 2s, big hitters can double-move and double-attack on the first turn with between 3-5 guaranteed extra damage (cards saved from E1), then Blood Rage for an extra two attacks to boot (possible with Expert Blow still in the mix). We lost all four heroes to Ice Wyrms (who have no innate surges) turn 1 of E2 in Ritual of Shadows, as OL goes first in that one.
A dirty mind is its own reward.
Said hero also loses anywhere from 40% - 90% of his life, depending on how lucky you we're rolling two red dice. OLs tend to enjoy pummeling heroes with low life - easy card generation.
Usually, "hero whack-a-mole" only happens when one of the heroes gets separated from the group. Part of the strategy of the game for the heroes is in weighing the costs and advantages of splitting up. Generally speaking, your heroes are at their strongest when they are all within three spaces of one another. However, it may be necessary for the heroes to go off in different directions in order to complete the scenario objectives. It's up to the heroes to make that decision, and making tough decisions is part of the draw of board gaming, in general.
Lonewolf, you simply must try the second half of the encounter. The first half of this one is heavily weighted towards OL, but in my game even though the OL got almost all the crops, the heroes obliterated Splig in the second half. These are meant to be full quests, not individual scenarios.
I agree with others that careful use of surges and fatigue is key, and that as the campaign goes on the heroes get more powerful. In my experienced, the game has been VERY balanced and every quest has come down to the wire. Either side could have won since both groups were playing well. Any mistake can cost victory.
Note specifically that knocking all heroes out isn't always a help because they can use a full turn to recover. Both sides should go for objectives, not just a total party/monster kill.
Since its release with 1st ed Descent has always been brutal. Sometimes with quests that were nigh unwinnable for the heroes (think Well of Darkness and even worse Altar of Despair). It is a strategically involved game where either side can win and mistakes can be devestating. I have always thought that is one of Descent's defining characteristics, and one of its greatest. When the heroes win in Descent it really means something. 2nd Edition, from my experiences, has made it somewhat less difficult for the heroes (though not by much :D). In particular, the reinforcing rule is much easier to plan for than line of sight spawing with multiple enemies. Ultimately though, my point is that to enjoy descent, the players/overlord have to be able to accept crushing defeats every now and again without all of the QQ. Otherwise, it won't be terribly enoyable. Which is a shame, as IMO Descent is tactically a tactically rich, challenging, and rewarding game. Just some thoughts.