I was able to get all 4 kills in mission one today, and IIRC all but 1 or 2 kills in mission 2.
I was not immediately able to reproduce all 4 kills in mission one so it must have been very RNG dependant. I had some savestates from the process and took a few screenshots but did not know exactly how i got to these savestates so there was missing RNG/steps that I may or may not attempt reproducing to varify if it's viable.
vague mission 1 sequence: 
save files to compare $$ between this stacked file and an average attempt:
this may not be RTA viable but I've been getting into this game pretty heavily the last week or so and so i figured I'd present my results in a public forum, since the records are all using the 3 kills strat from gfaqs.
Additionally, in regard to maximizing your XP gain you should always be using the strongest attacks available, even when the enemies HP is reduced.
I.E. Fireball will grant slightly more XP than Str. Slash, Str. Slash more than Slash, Slash more than kick, etc.
example: 
lasltly, in regard to the "99% miss rate" there is occassion when the RNG forces a miss regardless of context (target, attack, position, etc). the only way to change the RNG state (conjecture) is to attack (and miss) or use a support skill.
idk how active this game is but I saw schalakitty's run from last week on gdq.
Thanks for the insights. In my testing, I had once stumbled across a slightly different line of play that grew out of the basic 3-kill strat. I think it involved both Kamorge and Ione missing 99% attacks, but it did result in a 4K for Bastion.
I did not know about the bonus XP for overkills. That is actually a really big deal and probably changes the run significantly. Seems like a basic thing, but wow, thank you for pointing that out.
As for the RNG-state thing, yes, I think you’re right. The game has determined what you’ll roll on the to-hit chart before you go, so backing up and picking a new attack, different position, etc, does not change if you hit. Not relevant for most gameplay though, at least as far as I have tested. Is relevant for save-stating though.
Yea, the EXP awarded for an attack is depending on the % of maximum HP the attack would've dealt, even as an overkill. Which direction you attack from affects it as well.
Here's a link to my original spreadsheet that I used when I routed my initial Kingdom run: https://docs.google.com/spreadsheets/d/19J8oJdJYqD0fCllVI02g8L6XbechTaG8B_u_B1L4b6g/edit?usp=sharing
It contains ATAC stats, ability requirements, EXP modifiers (level discrepancies, positions), and some more stuff. Since I'm currently in the final year of university and working on my B.A. thesis, I'm not able to do much, but maybe you guys can find some use for it.
