For Humans, I find Silk + Crystal to be *much* easier than Plank + Elixir...
Paladin may not be the greatest, but he's still far, far better than the useless flaming trashbag that's the stupid Mortar.
If you don't have access to Orc Strats as a human, you're going to have a very bad time of it in the Plank/Elixir since Mortars will seemingly routinely lose event against the units they supposedly hard-counter.
(mostly because they do jack-all for damage... again, it gets a lot better for a human player if you've got Orc Strats and/or the Brown Bear)
I did mention that the order will shuffle depending on other factors. Mortars and Frogs in particular benefit massively from damage increases. With Frog Princes and a Fire Phoenix, Elixir is arguably the easiest province to fight in because you can simply kill or severely weaken everything before it ever makes it to you (the fact that Frog Princes are the only heavy range unit with a debuff that increases damage taken rather than reducing damage dealt is huge here), and that's the underlying balance issue with Elvenar's battle system as a whole. It's incredibly skewed in favor of range, almost to the exclusion of any other factors. It doesn't matter if your units are strong against the enemy units if they're still killed before they get the chance to attack. That's why the most recent Crystal tournament caused this thread to pop up. Sure, there was also a fairly significant bug in the game, but replacing the enemy Cannoneers with Frog Princes really hammered home for a lot of people just how imbalanced the five unit types are.
Instead of trying to be general, I'll go ahead and revise my difficulty ranking based on what each tournament costs for me to fight through:
Scrolls > Dust > Crystal > Silk > Gems > Planks > Marble > Steel > Elixir
I pretty much gauge tournament 'difficulty' by what round I can make it to before I start losing full squads of units in a battle. For Steel, I can make it up to round 4 before this begins to happen, and it happens first in the encounters which consist of 4 Thieves and 2 Thornrose Mages, the 'worst' battle in a tournament that favors light ranged units. For any of the 4 tournaments that revolve around using melee units, I'm already seeing full squad losses just in the first round. The others fall in between. Gems has been sliding backwards towards the more difficult end because I'm still stuck using Priests, but upgrades from the last 3 chapters have made other tournaments a lot easier. Marble's gotten Blossom Princesses and even Forest Wardens, Planks got Senior Orc Strategists, and Elixir got Frog Princes.
Anyway, perhaps my posts are long-winded but so many people don't bother with really looking at the game's battle system that I feel it's necessary to explain things in a little more detail.