No, that is not normal, it's an unusually high kill rate in my experience. When I've been in parties that hit those levels, there have usually been one or two kills per campaign that require resurrection (though more close saves with resurgences and whatnot).
It may be due to bad player tactics, weak characters, or the GM runs things tougher than the average GM - either "higher CR" or just "better", some GMs (like myself) manage to squeeze a lot more kill per hit die out of opponents. Sometimes a GM runs a harder game, which is fine.
But of course the thing about high level D&D is that you have multiple levels of defense against death, so as long as it's not impacting the fun, there's nothing really wrong with it. You get to play croquet with the angels on a regular basis, heck, that could be part of the plotline (Order of the Stick, anyone?)
If you do want to do something about it, and don't want to embrace character optimization, I would review the results of combats with the rest of the group and see if the deaths were preventable - do party fighters repeatedly charge recklessly into combat, ruining the ability of mages to area effect; do you get spread out all over the battlefield where the cleric can't effectively help you when you're in trouble; do characters not participate well (the tank fighter who hides in the back, the cleric who's too busy fighting to heal)?
Tactics In Depth
I had to coach one player group that was getting demoralized about how tough combats were going in one D&D campaign. In general they didn't have any coordination - they'd open a door and see bad guys. One guy would run in (before the mages could cast damage or battlefield control spells), others would back off, people didn't have mobility or line of sight. They'd decide to run, except most of the group would retreat, but one guy would want just one more round of full attacking, and then be cut off, and then half of the rest of the party would keep fleeing but the other half would come back and help them, and get cut off themselves... I taught them to do simple things like NOT go in the door, but back off and form an inverted triangle around the door and let them come to the party (for dumb melee opponents of course) so that they are the ones getting surrounded and losing actions and being separated. One PC that was taking on a leadership role took it upon themselves to "call the shots" in combat so that a retreat or attack was performed by the whole group. There's a lot of D&D-specific ticky combat tactics stuff, but I always see the greatest difference being made by system-independent "having your crap together" kinds of basic techniques. Have patience, don't get split up, coordinate maneuver to take the fewest attacks but deliver the most, concentrate fire.
I remember in one five year long 2e campaign I ran, the team leader went so far as to run IA drills and basic response plans. When they would come into contact, there were set formations they would move into automatically. The shouted command "Blue" told PCs to close their eyes to avoid the mage's Color Spray. The only PC death in that campaign came from PC on PC action.
I would do this with roleplaying and whatever social combat rules your system uses. The level of detail is determined by how important you want the trial to be in your game. If you are not using a system with social combat, you can also model it with "chase" or "extended contest" rules, anything where success accrues over time can probably be kit-bashed into doing the trick.
My problem with Jenga is that you're not building something stronger as you progress, you're making something weaker. It just completely goes against the thing you're modeling.
Here's how I would handle it:
- Determine the skill of the lawyers involved. Or the skill of the PC and NPC who will be the primary speakers at the trial.
- Determine the number of successes required to constitute victory.
- Ask what the players want to do in terms of preparation. Typical activities might include:
- Gathering additional evidence
- Tracking down witnesses
- Bribing, intimidating, murdering, and otherwise influencing judges and / or jurors
- Studying the law
- Finding precedents
- Tracking down the real criminal
- Getting someone else to confess
- Convincing frightened people to testify
Each of those activities could be an encounter or string of encounters. In the run-up to the trial, decide what the PCs get for succeeding or failing in these efforts. Smaller benefits could be modeled as skill bonuses, while larger benefits could be "free" successes to apply.
So if most of the PCs go out to convince people who can confirm the accused's alibi to testify, you can play that out. When they succeed give them a bonus to one or more "lawyer" skill rolls for use during the trial. Meanwhile, the (presumably imprisoned) accused PC is trying to track down the real killer in his prison. If he can get the guy to confess, you can grant them some number of free successes, based on how credible the confession is, etc..
In this way, you keep everyone engaged in the pre-trial process, and everyone gets to see their contributions contribute concretely during the trial.
Best Answer
Generally speaking, larger numbers of enemies (not tougher enemies) and better tactics.
It's a common tactic to just increase the stats of existing monsters or to choose higher CR creatures for them to fight, but that has a few problems. First, no matter how high their attack bonus and damage get, a single monster only gets one turn per round while the PCs get much more than that, so they can often just whittle the monster down just through sheer number of actions. It often ends up being a case where in order to pose any sort of challenge the monster has to be capable of one-shotting PCs, which isn't particularly fun. Instead, increasing the number of foes, even relatively weak ones, tends to make for a much more even fight. The enemies get to act more, they can gang up on characters and they're much less vulnerable to single target save-or-die spells.
As for tactics, having enemies play smart (or at least cunning) can make a huge difference. Since the PCs are playing spellcasters, have enemies close the distance with them (even if they have to take attacks of opportunity to do so), have them grapple, blind or deafen casters, make them spread out so they're less vulnerable to AoE attacks. Use stealth, have the enemies set alarms and traps, have them use buff spells or potions if they see the party coming. Have them flank and use the aid another action against high AC characters. These tactics aren't appropriate for mindless or particularly dumb creatures, but anything else should be doing their best to negate any advantages the party has.