The win condition is pretty basic, deal enough damage to your opponent. Theres no real gimmicks here that can be disrupted and ultimately cost you big.
However, there are numerous ways to improve the win condition.
Herald of Secret Streams- This guy here makes any creature with a counter on it unblockable. Merfolk just happen produce counters and work well with other cards that do so.
Metallic Mimic- I have had a love/hate relationship with this card because its super hard to keep on the board. If your opponent knows whats coming, you can bet theyll save removal for him. He produces counters for all other Merfolk entering the battlefield after him.
Vineshaper Mystic- This card is just simply awesome. It may not look like much on the surface, but it provides a decent blocker and 2 counters. Solid 3-4x.
Shapers of Nature- In a pinch this guy can turn counters into card draw, he can even add counters. He is definitely a great addition to the deck. Solid 4x.
Jade Guardian- This card has saved me towards the mid to late game. Hes a hexproof counter generator with evasion. He isnt a 4 of, but having him in the list is a good idea.
Verdurous Gearhulk- This guy wins games, he comes in as a 4/4 body with 4 counters you can distribute amongst your other creatures or just all on him. He cost a little too much to be a 4 of and you dont want to be bogged down with unplayable creatures in the early game. I decided 2 fit the deck really well.
Kumena's Speaker- A one cost creature who is a 2/2 with an Island or other Merfolk out is just a solid card in a tribal mashup. In the later game if you have Metallic Mimic out its a 3/3 for 1. Solid 4x.
Kopola, Warden of Waves
- This card is a nightmare for control decks that rely on heavy removal, and you should consider an extra one in the sideboard if thats your meta. Granting a form of evasion to your creatures is awesome, but at 3 mana with a solid body this card is just essential. 2-3x.
Nissa, Steward of Elements- Nissa offers Scry, tutoring, and ultimately an extra creature spell for free. The best way to use her is to load the battlefield by playing her when you have a surplus of mana and blockers. If you dont, then get her out for cheap and use her to get ahold of the spells you need. 2-3x.
Heroic Intervention- This spell doesnt just target one, but all of your creatures. It makes them hexproof and indestructible. In a deck reliant on pure creature power, this helps evade boardwipes and removal that could cost you big.
River Heralds' Boon- Two counters for two mana added onto up to two Merfolk. Cut and dry, but it works better than any other card. Oh, its also an instant, so you can throw the counters on to void a burn spell too.