I have two questions.
1) If I defeat a monster that has "Battle: DESTROY A HERO OF STRENGHT 4 OR LESS", and I have a hero of strenght 3 with a food that gives him strenght +2, must I destroy that hero?
You take bonuses into account when applying battle effects, so in this case the food would save your Hero from destruction.
2) The Evoker ability says "Attack +2 from rank 2 or higher(he can't attack rank 1). Dungeon: pay 1 XP for magic attack +2 and light +2" If I use the dungeon ability can I attack the rank 1 (only with the attack bonus of the dungeon ability)? or can I only attack rank 2 or higher?
Be careful what you're quoting --- the card does not say "he can't attack rank 1". What it says is simply "ATTACK +2 against Rank 2 or higher. LIGHT +1 if you have a Light item." What this means is that if your party is attacking a Monster in Rank 1, the Evoker does not get his attack bonus. He can still contribute his Light bonus, if you have the required Light Item.