The Gem provinces and tournaments are considered difficult for elves and easier but still difficult for human fighters. Your troops will be opposed by troups consisting of heavy melee, heavy ranged and light ranged units. with an overall favour for heavy ranged units.

WARNING: Be aware of the mistwalkers, there initiative is unbeatable disallowing you access to the sneak-peak tactic for free.

The most logic choice for these provinces would be the Sorceress, since she is considered good agains both the Knights and the heavy ranged units. Unfortunatly experience learned that the lack of range makes her an easy target and she often just dies, I only use her when I got the upper hand and Sorceresses to spare.

The Blossom Mage so far has given mixed results, since we only have access to her first 1 star form, she is extremly flimsy, but similar to the human priest she does have range to her advantage. she has it's merits especially with certain terrains or agains massive numbers of Knight. but the moment she gets in range of an enemy unit, she is dead faster than a fly.

The best results I have had in this province is with a combination of golems and treants. especially on manual mode with very difficult fights ahead. Golems can handle mistwalkers wel and are neutral to the other heavy ranged when at the same time they have plenty of HP to catch a fair amount of beating. Treants are powerhouses with massive amount of HP and a good defence agains the heavy ranged units. When your Treant grows to it's final3 star form as an Elder Treants, 1 or 2 can usually handle any amount of heavy ranged enemies, allowing you to field 3 other units to deal with any "other" thread on the field

In some cases you need to kill the mistwalker as soon as possible, in rare cases cerberus with it walking range might be able to assist with this, but in most cases the golems will do a much better job so try them first. (mistwalkers usually run into the range of the golem before his first turn) The archer have much more use, some maps are heavy on Knights and have little to none heavy ranged, they also do not have a disadvantage agains the mistwalkers making them an excellent choice, they also work well with golems. Archer are also a good choice when golems are no option and the cerberus is still in his first 1 star form, lacking the movement nessesary to reach te enemy.

As this is a difficult province to deal with, dare to experiment to learn what does and doesnt work.

This is the human province and priests rule supreme, with his range and special powers he can annihalate a warband of heavy melee and ranged troups in an instant. Alternativly Paladins can be used, especially fights with a lot of heavy ranged units opposing them. At my low level experiment they seemed to be almost as affective as the priests, and sometimes even better

The Mortar is vital to het rid of those pesky mistwalkers who are able hurt your priests and paladins fairly hard.
If you have access to the Orc Strategist you can use him instead, he really hates mistwalkers and seems to be a better alternative
On fights without heavy ranged units opposing you, archers can be an alternative unit you could use.

The basic combination you shall use is either all priest, or priests with 1 or 2 cannoneers depending on the amount of opposing mistwalkers.