Nope, the cleric isn't going to be raising dead every thirty minutes, he or she will be doing a whole lot more interesting things than curing light or serious wounds every third round. Let me clarify what highly lethal stands for in this context. It means your character may die from a single hit, any hit is potentially lethal. So hit point heavy characters and their players may face something uncommon at such a level, sudden death in the first round of the encounter. Characters will have to use all their skills and players all their wits not to get hit. Characters are not pampered behind huge hit points, they rely on the real thing: real skill and real luck and divine assistance.
When characters rely on skill and not bulky hit point accounts healing doesn't occur that often. Raise dead may be called for every so often, but the cleric is by no means left unemployed. The cleric's role shifts from being a red cross ambulance to a true representative of his or her deity. Among other things he can provide a channel to the gods and through it be granted divine assistance in battle. This can be done in many ways aside from a + 1 to hit or + 1 to AC.
Take a moment to shift paradigms from a game in which battle goes on for many rounds to one in which the first round is the most important round. What would you ask the gods if you had to hit first and hit hard? What would you pay to get that advantage?
As a GM, consider that monsters will be as susceptible to sudden death as player characters. No more high hit point ogres and giants. Sure they will be more enduring, and cutting at their feet with little metal toothpicks may not be all that damaging, but they wont have the huge amount of hit points they once enjoyed. Fear will play a factor with them too.
So here are some spells I'd find interesting:
- Clairvoyance powers. Having the party's deity hand over some information on where the orcs are laid out and in what position will add an advantage when targeting that first fireball. The magic user can cast darkness and then, knowing where the enemy is beforehand, dispatch a fireball in that direction.
- Stopping hits. Characters may be blessed with a once a day or once an encounter opportunity to stop a hit before damage is rolled. Getting hit should be way more uncommon than it is in high hit point scenarios. So calling this one can be a real life saver.
- The classic, a bless spell or similar effect spell that grants the character a bonus to hit or better yet initiative. Hit first, hit hard and don't let them stand up.