The visible stats for the Casey Bat are misleading.

If you reach max level before the end game, you will have a bad time (this requires obnoxious grinding though, like going out of your way for 1/128 chance item drops and having bad luck).

There is a detailed walkthrough, a full list of item / enemy stats, and TONS of information at starmen.net if you get stuck, but try to play blind the first time and consider going back for 100% after.

I haven't played this in a decade but it's my favorite game of all time. The desert sucks a lot because heatstroke is such a punishing status effect. Make sure you get a lot of the items that cure it. Wet towels I think. When you take so much damage in a single hit that it would kill your characters, their hit points literally have to roll all the way down slowly before they will die, if you burn through the menu choices of your characters and nail them with a heal spell that would raise their life to even 1 before the dial rolls all the way down then you can save them from dying. Very handy and saves you lots of revive items and mp. Buy a lot of bottle rockets for Jeff. The biggest tip I could give though is make sure you are playing with the sound cranked. The music and noises in this game are made by one of the most renowned Japanese composers to ever live. I remember the sounds from this game so deeply that it is part of who I am. The runaway fives bus, the sound spot song, the beach song of summers. So many awesome sounds. I envy your new upcoming experience!

Despite joining throughout the first 1/3 of the game gradually your party members join at lv1 so you'll have to grind em up when you get them. You may need to grind the party in general for a fair game but it's not an extreme amount, the exp you get just playing normal is alllllmost enough but you may wanna grind just a level or two for a smoother time. Read the item descriptions and don't just look at the status changes, additional effects are only viewable in the descriptions. The Casey Bat is an infamous example. Also talk to as many NPCs as possible as often as possible. There's a LOT of dialgoue in this game and it's mostly very charming. Otherwise it's a pretty straightforward playthrough.

Try to be patient with the interface and inventory management.

Don't be stingy with Psi powers. Magic Butterflies are commonly encountered in dungeons.

Grinding is easier after defeating the boss of a Shining Spot dungeon. The enemies there run from you so it's easier to get surprise attacks and insta-wins.

You can only get the Sword of Kings before defeating the boss of the Stonehenge base. All the monsters in the area vanish once the boss is slain, and the weapon can no longer be obtained. That said, Poo is plenty strong without it - None of the super rare weapons/armour pieces are necessary to complete the game.

Psi Thunder is powerful but often misses. As long as there are at least four enemies remaining, it's guaranteed to hit at least one of them.

Managing your inventory by phone is a pain in the ass. If you visit Tracy at your house, you can swap items in and out of storage at will.

The hardest part of the game is before Paula joins your party. Don't lose heart if the game feels too tough until then.

You don't have to call your mom regularly to prevent homesickness. Homesick happens completely at random, whether you just talked to your mom, or haven't spoken to her for the entire game.

YOU CAN SOFTLOCK YOURSELF IF YOU DON'T HAVE A KEY ITEM ON YOU

keep the pencil eraser and bad key machine on you, just trust me

inventory management is a fuck (your gear is counted towards this) so be aware of what you have (should i eat that cookie i've been carrying around for 10 levels? do i have any items i could use in battle here?)

there are a lot of goofy items, some are useless and there are some that aren't useless at all but seem useless. a ruler is usable to size up your enemy, but it doesn't do anything, meanwhile a broken laser could be fixed by a certain intelligent party member. so keep that in mind and don't be afraid to look up shit.

there are condiments that can be put on food, iirc they're used automatically when you eat something, they're pretty powerful.

your get money from EXP i believe, your dad adds it to your bank account which you can access thru an ATM. you do not need to call your dad to get the money, he just tells you how much he gave you.

bottle rockets are really fucking good, esp the Big variety! save them for tough bosses!

The starmen.net guide is great, probably more in depth, but the classic Nintendo Power players guide from the 90s is pretty comprehensive and goofy and funny. it should be trivial to find a PDF of it

i will edit this or reply to this comment if i think of anything

It's okay to look at a guide if you get stuck, there's a couple places where it's a bit moonlogic.

Always use the latest gear, especially weapons, but avoid yoyos and slingshots.

The game expects you to grind a bit at the start, but this tapers off a lot once you have multiple party members.

The biggest places you can get stuck if you happen to do things in an order the game doesn't expect (open if you're stuck in a place for ages):

The waterfall scene is pretty tedious so just take a piss while you wait under it.