Switch out Jimitron for Mondevol. Mondevol is a good tank, hits decently hard, and magic+electric is a nice combo. Pair with Broomstick to eliminate earth weakness; this leaves it weak to only a few types, which are just bad in the first manamon game (insect in particular, for instance).

Considerations for a final team member:
Deciced: Ice-shadow, not insanely speedy but decently tanky on the physical side and definitely covers a lot of types with its STAB
Fiendour: Another semi-tanky manamon which, with Headstone Amulet equipped, has almost no real weaknesses at all; doesn't hit insanely hard but is no pushover, and has some nice HP restoration with Desoul
Mortrex: ghost-shadow; if you can get this final evolution, it's potentially worthwhile, as it's pretty speedy and gets Subliminal Radio Wave for confusion. It gets better in Manamon 2, but in Manamon 1 is not bad at all.
Frenrir: pure ice type, physically a bit frail but gets some nice moves plus a water move to handle two of its three weaknesses
Bellaard: sound-holy; you might think I'm nuts, but hear me out. If you can stay away from undead and ghost, you might be okay. Holy is a nice attacking type, and Bellaard does get Divine Protection, Sing and Subliminal Radio Wave. In boss fights, this thing can be a nice annoyer. There's better, but this isn't bad.

I've avoided stuff like Dussocket, Grandlan, Sacraloon and Remullion because you don't really want to double up on types if you can help it. Generally I choose Grandlan over Leonatar now, particularly if I have another Battlecry user for the Irroadium fight.

Yeah. If you bring something like Frenrir and Martiagle to the Irroadium fight, you'll crush it. And there are no other really difficult fights in that game.

There are a few, yup.

This isn't exhaustive, but here are a few I can remember:

1. In manamon 1, magic is super-effective vs. shadow. In manamon 2, shadow is immune to magic
2. In manamon 1, steel is not immune to poison; in manamon 2, steel is immune to poison
3. In manamon 1, undead is not weak to ice (IIRC); in manamon 2, undead is weak to ice
4. In manamon 1, undead is immune to poison; in manamon 2, undead is resistant to poison
5. In manamon 1, plant and insect are neutral against ice (IIRC); in manamon 2, ice resists plant and insect instead

It honestly doesn't much matter, as long as you get your train points eventually. Manamon 1 doesn't have items to do this except for life tonics. Just add train points according to what you want the manamon to do. For instance, don't give Leonatar special attack, don't give Garganchan speed. Stuff like that. Play to the manamon's strengths and you should do okay.

So...let's see here.
Mondevol's defenses aren't sky high, but take a look at that HP stat. also bear in mind you might have bad defense stats, so that's important. This thing can't have everything. Mondevol is sort of like Fiendour, in that it's meant to be tanky and stick around awhile with pretty high HP and decent defenses, as opposed to stuff with better defense but awful HP.
You just hit on another reason that Bellaard is useful. Being able to wreck holy is nice. Now don't try and rely on it to kill Mortrex though, since Mortrex is faster. Arguably, Sacraloon is just a better holy type (it's one of the best manamon in this game) but at the same time, you've already got an air type.
If you grind in the proper spots, it is not at all difficult to stay ahead of the curve.
Manamon 2 is more difficult than manamon 1, but only because there are gimicky bosses to play with, and one of them is actually pretty early on. The actual tamers/stadium leaders are, if anything, even less memorable in this game than the first, I'm afraid.

Just FYI, it's Batter Branch, not Ghatter or whatever you said.

Martiagle's ideal moveset is something like Batter Branch, Cyclone Claw, Fly, Hone Blades and Obscure. Obscure lets it get more and more evasive, Hone Blades makes its attack go through the roof, the rest is self-explanatory.
Purple Ring is really, really niche. At higher levels it's not bad, but generally it's worth just hanging onto and then probably forgetting about.
Mondevol's best moveset is something like Aura Master, Superspark, Quicken, Vulnerability Scan, Laser Arm (does it get this?), Thicken and Stun Volt.
Guerrerrol works best with Power Kick, Gammadrain, Slash, Implant and whatever else.
Leonatar...make sure it keeps Battlecry. Also Earthquake and Slash for STAB. Charge of Fury isn't bad, but you probably won't be using it a ton. Maybe keep Roar around as well.
As long as Fauntozaur has Drench, AquaZap and maybe Pummel, you'll do fine. Also, if you want to make it an absolute beast to kill, get hold of a couple of speed potions and give it Dissonant Chime. Jack defenses, drop speed, then use a speed potion or two for an instant tank. Generally I don't like Dissonant Chime, as it's a really great way to let your foe hit twice in a row where he couldn't before, if only the once, and this game's meta is hyperoffensive and not nearly nuanced enough.

Seriously though, stick Frenrir in your last spot and this team will absolutely destroy the main game. Make sure you're around level 50-52 before hitting Shadow Kingdom, and don't be afraid to use the walkthrough. That's why I made it.

So I realize now why I was so unimpressed with Mondevol, and that was mostly because it was 10 levels lower than everyone else on my team. Now that he's higher, I can deffinitely tell that his defenses are really good. Laser Arm seems to be a good move that also relies on his physical attack which he is better with.
Now I'm attempting Shadow Kardom, and I totally understand now why it's super hard. Those shocks are avoidable, but really hurt when you don't. Honestly, my least favorite obstacle in the game are those guards. I'm not really sure what their sight range is, so I basically BSed my way through the Shadow Warrior Hideout in that sense. I can avoid the flames, the shocks I can deal with (I'll learn to anyway) but the guards make that part not fun to go through.
I honestly love Snowball on Frenrir just because it recks things if you keep it going, sort of like the Whitney Miltank Rollout strategy from Gen 2 of Pokémon. That I think would seriously help against Irroaddium, slowly adding up damage over time to counteract his healing.
I put the power bracelet on Martiagle because if it kills things in one hit, crits won't even matter.

Well, I just beat Irroaddium, and honestly, I had trouble at the beginning because I forgot that I overstocked on revives. I learned that my level 52-ish Frenrir could survive 1 Flame Blizzard, which allowed me to use Mondevol as the healer to keep him buffed.
Hardest part of Shadow Kingdom were the guards, because their collision detection is super wonky and I still don't understand it. Luckily I only got insta-killed like twice from Final Oblivion.
Literally every stadium after the first two have been cakewalk, and I am confident Electra will be super easy as well.
I now understand how good Frostbite is, as it did a solid 95 damage to Irroaddium each time.

Resurrecting this topic because I have a question.
So I finally beat Ascendance Path (I hate that place more than Shadow Kingdom, and that's saying something), and I've already seen the Vibrogen rocket laudch.
I shouldn't have been able to because my manamon's averages are Level 60-ish fighting against Level 70. Is there a better place than Crescent Walk to grind that 10 level gap?
My team right now consists of Fautazaur, Martiagle, Frenrir, Mondevol,  Guerrerroll, and Leonatar. I'm honestly less than impressed with Guerrerrol's fighting ability: he gets shut down quite easily (I can barely use him against Washington, who is water-type), and Monevol still doesn't seem to last as long as it could (against something like Enchandithol with Meteor Rush). Any tips on how I could go about grinding, or would grinding against the master's stadium work best?

True point. Honestly Mortrex is super annoying to deal with unless I set up Implant on Jimitron and switch out to Martiagle with Rubber Gloves equipped (honeing blades until right before Jimitron dies) and then Flying to avoid Psycho Trick.
I realize now how helpful speed reably is: Dragomier can be OHKOed by Leonatar only if I Quicken him twice. If not, I most likely will die.