Killing machine vs great dragon *end-game spoilers*

I've never leveled up either one of them, because I've not gotten to them in my file that has all the monsters recruited and I'm nowhere near there yet in my uber-completion file, but the thing to consider is not only how they are right now at whatever levels you have them, but also the maximums and how long it takes to get them there.

Both of them take around 2,000,000 XP to get to their maximums, but that's L30 for Killing Machine and L60 for Great Dragon. The levels are thus spread out much more for Killing Machine, so the growth will be in larger chunks less often. Their final maximized non-seeded stats are actually reasonably comparable to each other. Without taking into consideration any weaponry or gear, they both end up with maximized strength at 255, and the rest of the stats are not too far off each other, excluding wisdom which doesn't matter for non-casters anyway. GD's wisdom is much higher for what that's worth, which isn't much.

So I'd say if your great dragon is already many many levels above the killing machine, you might not gain enough advantage from KM's better gear choices because of all the catch-up time you'll need to make the switch worthwhile. Up to you. Beyond doubt those are two of the game's better monsters at any rate so it's not like you'll be making a bad or wrong choice either way.
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Frazee is barking weirdly!

Final maximized HP for each are GD: 511, KM: 450, so they both end up great tanks with a slight but not overwhelming advantage to GD. Neither has any MP because GD's AOE attacks are all breath type, not actual spells. Forgot to mention that in the above post.
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Frazee is barking weirdly!

Well, Slime Knight can potentially level all the way up to L99 with 500 HP, but it takes him 6.8 million XP to do it, so chances are you won't get there in a standard playthrough, even close. He also will get to maxed 255 STR by then, but with much lower innate resilience, 120 to KM's max 255 when both reach their caps. SK is certainly very very viable, especially for an easy recruit you get right off the bat, so for those that don't want to do the hunting grind, he's looked at as just about the best and most indispensible monster recruit.

The three that are easy recruits that are all through-the-postgame viable are SK, Golem, and Orc King. Those three are pretty standard monster recruits in games where people aren't trying for monster recruits, and are almost as powerful and viable a team when leveled as most teams that have some rares. For your purposes, though, if you don't feel like you need the extra healing, then yeah, KM is probably a little better for those reasons.
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Frazee is barking weirdly!

I've got mine to around L50, and he's decent, but outshined by the best of the random recruitables. I kind of like him for a pet, though, since he's been there since childhood, so there's a bit of sentimentality working for him. Me liking to use him comes from the same mindset that causes me to always marry Bianca.
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Frazee is barking weirdly!