Assuming you came over from D&D - one core difference is that Ars Magica isn't really "balanced". There is no standard CR or character level, no standard set of stats that will pose a predictable level of challenge for character with amount X of experience. Instead, it's all done by tailoring the challenges to the party.
You should build up the monsters based on how you want them to function in the game. Some tips (all just my personal opinion):
- Magic Might is most important for setting Magic Resistance (=Might Score), especially against young magi. Look at the magi that you plan will go on this adventure and set the Magic Resistance to a level you want it to be at. A major threat might have enough MR to make the main-combatant's attack spells fail without luck/exertion, for example, or a grunt might have enough MR to occasionally be felt by the second-tier combatants. Remember the effect of the Supernatural Aura on both MR and the magi's penetration. You may want to give some monsters tricks or capabilities to increase their resistance to some spells; for example, battling fire elementals with immunity to fire might force your Flambeau to use his secondary attack spells, allowing perhaps to set a Might that will be relevant to all combatants. Or the use of illusions can keep low-Might foes challenging.
Might Pool (=Might) is only rarely relevant, in my opinion.
Raw vis = Might/5 is standard, which means that fighting lots of monsters will give your players lots and lots of raw vis! This can get out of hand if you're running a saga where vis is a significant factor. Consider how you want to handle that as an SG. Such sessions might simply be rare enough to not matter that much, or you could arrange for the vis to not be collectible for some reason (the bodies fall to the lava pit below...), or you can institute a house rule to lower vis collection (e.g. 1 pawn per 10 Might, or no raw vis from monster-slaying at all, or harvesting requiring a seasonal activity, or so on). Or you can just accept this as a source of vis in the saga, and plan the saga accordingly. Whatever you decide, be sure to realize in advance that in Ars Magica killing monsters and taking their vis can be a major element in the game, kinda like killing monsters and taking their treasure in D&D.
Set the creature's attack and damage against your PC's defense and soak. Keep in mind the shield grogs, if you guys are using any. Also keep in mind that combat in Ars Magica is quite deadly - wounds can take forever to heal, and a few bad rolls and you're dead. So keep bad rolls in mind when you set the attack and damage. Assuming some bad luck for the PCs, what will the monster accomplish?
Set the creature's defense and soak against your martial PC's stats. Remember Soak applies against some of your PC's attack spells (e.g. fireball, erg, Ball of Abysmal Flame). Again, consider the creature's role - do you want to set up enough Defense for him to shrug off the grog's attacks? Do you want a Defense + Soak small enough so that the companion Turb Captain can punch through with a bit of luck and some exertion and/or Confidence? Remember the death spiral - once someone does wound the monsters, getting more wounds on top of that becomes easier.
If you have Trained Groups in your game, take that into account. This can really change your PC's stats.
- Now that you know the basic stats you want to reach, think about how to do that. You can play around with Size, Characteristics, Combat Ability, and Weapon Bonuses, but you can also tweak with Virtues and Flaws such as Affinity with (Ability) or Tough.
Weapon Bonuses for monster weapons are in supplements. But you can just assign number as you wish, based on the core book's weapon table.
Increasing Size affects combat stats as described in the Beast of Outlandish Size spell.
Modify by Combat Ability (usually Brawl) and Characteristics to get the Attack, Damage, and Defense (and Init, if it matters) to where you want them to be.
Add things like "Tough Fur", or "Scales", to arbitrarily increase Defense; these are called Qualities. Official ones are in the supplements, but you can just make them up as you need them. You can add Qualities to modify everything: "Herd Beast" can increase Awareness and add to Quickness, "Dragon-soul Body" might add +10 to Soak, whatever. Just add Qualities (or Virtues or Flaws) as you wish to modify the stats as you wish. There is no need to balance anything, this is an NPC not a PC.
All of this is very complicated. But if you have the time to invest, it's the best way. If you don't have the time - I recommend just looking quickly at the PC's stats and picking up some numbers for comparison ("Hmm, let's see, the Flambeau's Penetration 15, and the shield grog's Defense is 18, right..."), picking up the animal from that Book of Mundane Beasts that matches most closely, changing the Size to fit, and then quickly, roughly and arbitrarily changing the combat stats to fit those key numbers you picked up ("OK, so I'm gonna set Might 10 so the Flambeau can always penetrate, and increase the Attack to 20 to let them hit the grogs; right, that's done"). You can justify that by adding Qualities, changing Characteristics, or whatever if you want to; but you can just leave it as arbitrary changes too.
- Another major tip is that magi can get very powerful in combat, if they are designed and advanced to do that. So much so that Ars Magica is often a bit like Epic Levels in D&D - the question isn't whether the PCs can kill X (of course they can), the question is should they and what the repercussions would be. So get used to the idea that if you saga develops, and assuming you stick to the core rules and the game is combat-oriented, you're likely to soon find that you can't really challenge your PCs in combat. Not without pulling out a lot of non-core material (yours or the splatbooks), other Hermetic magi, or house rules.
That's what I have. Have fun now!