Why why why why???
I swear that when I’m level 60 I’ll still be reviewing 上げる, 上がる, 外す, 外れる etc … I constantly get them wrong even when I feel confident about the answer … why you do this 日本語??? …I mean if it followed a pattern every time it would be easier (yes I know す is usually transitive but I’ve hardly come across verbs that end in す)
Does anyone else have a similar experience? If so how do you overcome this? …please don’t just say memorise it 
Okay, if you don’t want to memorize, I’ll give you the other typical answer to the problem: Read a lot 
Not even joking, seeing them used in actual contexts a lot makes it super easy to remember which is which for some reason. It requires you to also work on other important skills for your Japanese, like grammar and non Kanji vocab, so it’s a win in all directions.
For actionable advice for right now, I’d add to not stress about it. Let them be leeches for a while, and let them make you smile when you seem them. It will sort itself out naturally at some point when you progress.
Thanks for the advice downtimes, that’s probably what I’m missing …context, maybe I should make up some sentences to help remember them. 
…sometimes I find the context sentences provided to be either too odd or too complex to remember …well at my stage anyway.
They’re still leeches for me 
When you have vocab with similar meanings that you can’t work out, build a sentence including them both. Use that sentence along with your kanji mnemonic. Problem solvederino.
This is a problem that is only encountered when you try to learn vocab in isolation.
there are some rules, though - the two I remember are that any verb that finishes in aru (e.g. 上がる) will be intransitive, and that any verb that finishes in す (e.g. 出す) will be transitive. I can’t remember the cited source but this always seems to work. Doesn’t cover all cases (e.g. for 入れる vs 入る you are out of luck) but it’s something…
I suppose I should ask, do you need to know how to distinguish and articulate transitivity for a college course essay/exam, or are you trying to learn Japanese so that you can understand and use it? In the former scenario, knowing transitivity is crucially important, and in the latter scenario, it is the furthest thing from useful IMO.
There are something like 9 different transitivity verb pairs and with all of them but one pair, one will always be transitive and vice versa. The one pair that doesn’t quite follow this rule is is the く、ける pairing. For that one, whichever “feels” more like the main transitivity for that verb will have the く ending and the one that feels like the secondary transitivity will have the ける ending, i.e. think about what verb transitivity the ancient Japanese would have come up with first when they were coming up with the verb pair for the first time.
In general, Wanikani will have both verbs to a pair for any given verb pair in their massive list, so when you get a lesson for a verb that you haven’t come across its pairing for yet, it would do good to immediately search for its pairing in Wanikani.
My partner is Japanese and I’m trying to learn how to better communicate
If you ask your partner if a verb is transitive or not, would they be able to answer promptly and correctly? I suspect it’s not important to the learning process.
Good call … but like a native speaker they probably don’t think about it, lol as a native English speaker I actually have to think about grammar points when asked, I don’t just have the answer straight away (plus I didn’t pay enough attention in college when I was learning grammar … the whole “why do I need to know this, I can already speak English” … stupid 13 year old me … if only I knew then what I know now 
) … Skin Meat (Ironically …see what I did there, lol) I’ve learnt more about English grammar from trying to learn other languages.
Yeah, the thing that worked for me when I was having trouble with these was remembering little sentence fragments, so not just the verb but something with the particles. If you can remember that it’s ドアを開ける that’s what you want really, rather than trying to associate it with an English phrase or with a label like ‘transitive’. Eventually practice in either input or output will solidify this.
On the other hand if you ask them “is 電気をつきました right?” I don’t think any native speaker would have trouble saying “no”. I think that’s really what we’re trying to learn and internalise here.
Might be a bit corny, but when my house gets flooded out, as a general rule of thumb: 水位が上がったら声を上げて！ When the water level rises, raise your voice! It doesn’t have to make sense, as long as it helps distinguish the problem words.
Right. The scholarly verbiage in articulating the rules of grammar isn’t needed; just a basic understanding of how they are used in a sentence.
One place a person can see more about this “ある” and “す” connection is in this subtitled video by Cure Dolly:
I still confuse a lot of them, my eternal leeches 
I used to worry about Japanese’s transitivity being difficult to master, but ever since discovering these few patterns, it’s become very easy for me. I found all of this through that Cure Dolly video that @ChristopherFritz mentioned earlier:
Verbs that end with ～ある sounds are almost always intransitive, just like ある is intransitive.
Examples:
Verbs that end with ～す (or ～せる) are almost always transitive, just like する is transitive.
Examples:
These are all vocabulary words from the first 10 levels in WK. In those 10 levels, the only exceptions to these trends I could find were 語る and 配る; both are transitive, BUT they don’t even have partner verbs in the first place, so it’s effectively a non-issue.
Lastly, many verb pairs include a word with an ～える sound, like 止める, 当てる, 終える, or 出る for example. These verbs just flip the transitivity of their partner verb.
Tagging @AnimeDayTrader, @evandcs, hopefully this can help them, too 
This is the article version. I recommend reading it. It will change your life
I’m inclined to wholly disagree with this, unless you mean to say you’re okay with saying things like “He falled the ball” (instead of “dropped”) or “The man killed” (when you meant to say he died). Besides that, word choices like 見つけた！ vs. 見つかった！ are often the difference between “I found it!” vs. “I’ve been spotted!” Sure, you can rely on context a lot of the time, but there’s already an abundance of ambiguity in Japanese without treating transitivity as an afterthought or guessing game 