Mages use intelligence for attack power, for mikos it increases healing, Tacticians use it to get a bigger auto-buff at the start of battle and also increase buff strength, it can also be pretty usefull for ashigaru because it increases the guard percent you get when using the skill. Rizna is classified as a mage so intelligence would increase her fire blast attacks.
Edit: I also saw your other post above showing the game map. at that point you might want to first take the Akashi province, then take Takuga. Takuga is important if you want to go through the uncursing Mouri storyline instead of just conquering them normally. It also gets you some good commanders. Or you can try and fight Takeda, I actually found them harder than Mouri. Honestly in your first playthrough its really hard to take either Mouri OR Takeda, usually the monster army will show up before then.If you are avoiding Akashi because of the gourds you could try and take the first mouri province then take Takuga but that will leave in in a war with a lot of enemies at once.
But looking at the provinces you have at this point you should have Kenshin, Urza, and Natori in your army which if deployed together should be able to beat most battles. Every battle Kenshin is in she has a passive ability that reduces enemy action points by 1, If you upgrade Urza to get her accurate shot ability that ALSO reduces the action points of every enemy by one, not to mention that her buffs are also extremely good. With both Kenshin and Uruza you are taking away 12 turns of attacking from the enemy if they have 6 units. Natoris special ability will kill 30% of every enemy unit in the battle. All these abilities are extremely good and when combined can take on really hard armies. Also if you bring a monk with you that has convert action you can use it on Natori or Uruza to use those special attacks AGAIN.
