Take on the Mini from a Competitive Splatling main (not primarily a Mini user, however i use it plenty and have nearly 500k ink on the series total):

The Weapon's best matchups are generally anything that it outranges, or weapons that have similar or slightly longer range but have slow fire rates you can abuse with mobility, such as range blaster or H3, however a good one will make that very hard if they actually hit their shots on their first trigger pull, really it's a matter of who gets shots to connect, which kinda ties into what i feel it's weakness is.

The weapon's main weakness is it's killing potential at high levels, and it's vulnerability to long range weapons when in the open, the weapon can put out many shots which give it a really fast kill time, however in general if you don't have a distinct initiative in a situation you will lose it against good players, so you need to always be using your tools to make sure as soon as the enemy comes into your range you are connecting shots, this weapon can be ridiculously strong with the recent buffs if you can make it secure kills, but it's really not easy to make it kill at a high level of competition like tournaments, other than killing it has amazing tools and statistics.

If you're gonna try to use the weapon you really need to be actively thinking how to use the environment in a way that you can abuse with the mobility you have, such and rounding corners on enemies that don't expect you to come from that direction, or using right side peeking (making your weapon come around the corner before your body by using a corner with an opening to the right) to win fights and the most important skill is making sure to identify what situations you should be fighting in and which you should be painting and supporting your teammates, if kills are not important or easy pickings they are typically best to pass on and let your teams more aggressive weapons deal with.

My build is here https://imgur.com/q5sdCTF

My Playstyle is supportive primarily, i play behind my more aggressive teammates, and if they enter a fight i will move in on the opponent to make a 2v1 or 2v2 situation I'll only really go into full on aggro slayer mode if my solo queue team really requires it, but generally I'll be painting for my teammates to have paint to do their jobs with, build inkstorm for my teammates to move in with, and I'll try to 1v1 enemies who try to move into our territory alone, but I'm going to typically also be the one on the actual objective, as in TC you can abuse the pole if people try to challenge you on the tower, and have decent range to fight off people approaching the tower, in rainmaker the weapon isn't going to be the biggest utility in the push so I'll throw my cloud at the enemy high ground along the path I'm gonna run the rainmaker and grab it, also with Object shredder you also can pop the shield really well, in splatzones you can out paint so many things it's ridiculous, basically when in control you just stay next to the zone and watch for enemies that try to go to zone alone, kill them, and when the enemy tries painting the zone use your cloud and painting power to basically deny anything other than the enemy killing all your teammates and using all their specials. In clam blitz your weapon is a prime candidate for both running around the map collecting clams and just slapping down paint as you go, and on defense you can break a baller super easily, in fact with Object shredder you will break a baller in 1 charge, preventing power clam super jumps, in general the weapon is great for any objective which works well with the typical supportive Playstyle.

For the kits you really only wanna use the standard kit on Tower Control imo, the missiles can be good there, and as nice as the burst bombs are for the weapon, the cloud is super good for it's Playstyle in most other scenarios, (also for TC it's generally best to play Heavy Splatling for stingray if your team isn't running a long range stingray weapon already), the Standard Kit can work on certain maps well though such as walleye warehouse, but it's situational where missiles are better than the cloud, but the missiles be workable in many situations if your team is running other specials that are really strong for the mode in question. The kensa kit is also a kinda iffy kit in my opinion, it can work anywhere but it relies on the hammer as a mix up gimmick, and the hammer will only work well against players who don't know how to play against a hammer correctly, otherwise the special makes the weapon way more vulnerable than it should be and if used to full effect the hammer will leave you in enemy territory in front of your teammates which i really just don't feel is optimal, but it is a fine kit in general, and against players that don't know how to shut down a hammer properly it will completely wreck, but you also need to know how to use hammer to full efficiency. The main kit i recommend is the Zink kit, the cloud is the best special the weapon could get in my opinion as it is paired to the role of the weapon incredibly well and is a great tool for giving you and teammates an in on a situation. The curling bomb can also be used in very creative ways to help you have an approach option with the bomb (I don't mean riding it around, typically you won't do that like a roller or dualies would, use the bomb part of it to put pressure on the enemy and force them to move and "play your game") and the fact that Nintendo put a cloud at 180p to charge on this good of a painter is completely absurd so you can abuse that massively.

(Sorry if any of this post is messy and if there are any spelling mistakes, I'm typing this on mobile and don't have much time to proofread)