Hargrimm

The Navarre S-rank ring gives +15 crit when attacking with a sword. Would have to lose the Sniper +10 and go Wolf Rider or something for sword access but still net out a bit ahead. Plus of course Wrath inherited from Ike is free crit whenever you take some chip damage.

There is a dodge button that could potentially get you around without much left stick involvement, but that would be pretty laborious. There's a free demo available on the eshop, probably best to just download that and give it a try to see if it's workable at all for you.

I was able to replicate this with a Golem, was at 4/5 for the level 2 achievement, and when I first killed it the achievement immediately popped before the mission ended. Then I restarted, killed it again, finished the mission, and was now at 6/10 for the next level. So it looks like those forms of achievement progress are incremented immediately, without the mission needing to resolve. So technically you can always complete the entire achievement path whenever you encounter a single one in any mission if you're patient enough to redo it 10 times, interesting.