Like all good RPGs, Rune Factory 2 has some boss battles. Each of the four caves has it's own special boss. After you defeat the boss, you'll receive a fragment of the Stone Tablet of the Cave from the Boss' Altar. You'll also receive some unique recipes. If your levels are sufficient, then you shouldn't have much trouble beating the bosses. All of the bosses turn purple once they have reached fairly low HP
A good rule of thumb is to have a few Recovery Potions on hand. Obviously, you'll need a strong weapon. You can buy one from Tanya, or forge one for yourself. The Medication Spellbook is notably useful, as it can cure any bad statuses you receive. The other Spellbooks can also be used to deal some heavy damage to the bosses. Medicinal Herbs are also useful in the case that you can't use Recovery Potions.
 Seasonal Caves' Bosses
 Terror Tree
The boss of Trieste Forest. It can be found in Wooded Paradise. It's first stage of attacking is having thorny brambles sprout up from the ground, hurting you. It's rather easy to avoid them, so you shouldn't have much trouble dealing damage to the tree at this point. The next stage involves the tree hurling apples at you. The apples come in a circular pattern, so they are also pretty easy to avoid. After you have depleted much of it's HP, it will turn purple, a sign that it is about to unleash it's strongest attack. It will remove itself from the ground, and chase you. It moves rather slowly, so it's pretty easy to avoid. If your HP is still high, you can just continuously attack it from one point. Alternatively, Magic can be used to deal damage from far away.
The boss of Blessia Island. It can be found in the old ship's ruins. You cannot deal any damage to it if you attack it's shell, but you might be able to occasionally hurt it by attacking from the side. It's front is the only fully vulnerable place. However, if you are confident in your defense levels, then you can just stay at the front of it, and chip away at it's HP. It wields a spear in one of it's tentacles, and will occasionally attack you with it. You can do the old run away, run back, attack, and repeat strategy here. It will also occasionally throw barrels filled with explosives at you. They are thrown in a long arc, so you can avoid it by moving far away. It should turn purple after a while. It will then send out poisonous, puffy clouds. You can use the Medication Spell to evade the effects.
 Bane Dragon
The boss of Messhina Valley. To find it, go to the end of the bridge at the north of Messhina Valley. Once there, you will be teleported to a location that looks like the bridge you were just at: This place is just a palette swap. The only way to get to this area is by going to the end of the bridge, as there are no other ways to access it. It's first stage of attacking involves chasing you down the bridge, attempting to crush you. If your levels are high, then like the previous fights, you can stay at one point, and chip away at it's HP. It will lash out it's tongue at times. The tongue can paralyze you, so having the Medication Spellbook on hand is particularly helpful. It will also occasionally shoot out blue energy balls from it's eyes. Attacking it's front is pretty hard, so it's best to attack it from behind. After it's HP has gotten to a low point, it will turn purple, and start jumping while chasing you to deal more damage.