Alright, so I THINK it's okay to double-post if it's for a good and clearly helpful reason. I'm going to try and answer my own question starting from the top!
Okay so say you're like me and don't know what you're doing, but, you have FEBuilder, which comes with Event Assembler, but you haven't used that yet. You've noticed that in FEBuilder custom battles seem kind of arcane. I'm using FE7, you're probably using FE8 which will look different but our guides cover both so we're good. It's just important to know the systems are different.
First, we need pointers for the scripting for our fight! In FEBuilder we can find and kidnap code for fights but FEBuilder won't tell us much. It tells us a pointer and not much else. To make a fight, I need my own pointer.
This Disassembly Guide helped me figure out how to use Event Assembler, and now we can disassemble the event we want to put our custom battle into. Once we have our disassembled code, we can just make a space where we want our fight to be and put the code line,
FIGH Attacker Defender FightData as per Arch's guide on scripting battles for FE7. If you're in FE8, you'll find your syntax there too, and this and my first post has images of those posts for quick reference. You'll notice that if you disassemble an event and look at the FIGH codes, they are indeed structured like this, but they'll have crazy pointers like 0xCB29A0 (right...?) or just 8CB29A0, and Disassembler did not tell me where those pointers go or how to get to them. But we don't need to we just follow Arch's guide.
Although I have no idea what (lastAttack) means. I don't know what CAN go there or what SHOULD go there.
Avril here didn't know either, but this post can help us now that we have disassembler and can now learn on the fly. CT075 recommends putting "True" there, and that worked for me. It DID NOT let me get a scripted battle with the Fire Dragon working, that broke, but I was able to get a battle working between two units and put it into the game... barring some weird, weird jank. My battles have serious graphical glitches on startup, they kind of make the map freak out while the fight is starting, the opposing unit died and gave their death quote unwantedly, so I'm not sure how to perfectly recreate a scene like, Jaffar and Leila. It is at least more than what I had before and we can improve from there, I'm probably just missing something obvious I'll figure out later.
Here's the code I was using that "worked." Newbies like me, please note the "#include EAstdlib.event" at the start, we want that. Event Assembler wants that so it knows that AttackerHit and such are defined, and without it Event Assembler should treat AttackerHit and such as hot garbage nonsense noise.
So instead of a pointer like 0xCB29A0 for Jaffar vs Leila, I'm able to define my own pointer and name it whatever. Then at the end past ENDA we define the battle contents that our pointer FirstEwan contains.
Again I don't know what goes in lastAttack during "AttackHit(7,attackLast)", but the suggestion to just write "True" seems to compile fine??
And I think we definitely need the BLDT but I don't know what that does, I think it just signals the event assembler to stop adding definitions to FirstEwan.
Cool so we have a scripted fight for a pointer, we know where to point our pointer, we can name the pointer whatever we want and put our custom name into the main code, and it shows up in FEBuilder and... *kiiinda* works, barring graphics issues.
The last thing I wanted to document for everyone else, and especially because a little bit of help at this juncture would be nice~, but, ASSERT... I couldn't get that thing to work at all.
So I made it,
so that it won't run, because event assembler was refusing to, uh, assert it, I guess. But we want that!! Well I guess I have to do it the hard way, and if you're in my situation too, here's what I did.
I went into FEBuilder, went just, anywhere, and told it to allocate new space for an event. Then the offset for the new event? I just grabbed that. FEBuilder offsets our changes to events for us I'm guessing, so then I just use Event Assembler to assemble the event with our fight and its instructions, and we assemble it to the offset of our new fresh event. Essentially offsetted myself! Now the old event is redundant code so I'm guessing I can just... delete it? Or I could leave it there. I haven't used the REBUILD ROM buttom in the advanced menu yet so I don't know how safe that thing is, but I imagine if it works as intended that function will appreciate it if I delete unused events to make room and keep the ROM trim.
So those are all the guides I was able to find that were relevant, and I've now documented how I figured them out and most importantly, what order those guides could help me in, starting with disassembler. Once we can disassembler we can learn anything. But I'm also not sure I can learn much more on my own without help.