Mercenary right now is either last or 1st, with how the meta is revolved around rolling at 8. Two merc players in a lobby drastically improve your chances of you both going bot 4, your merc units are contested and then you can easily break each other lost streaks resulting in early cash outs.
You play for a good early/mid game and roll at 8, or loss streak with mercs and roll at 8, or yordles and roll at 8.
You are looking to level to 4 always on 2-1
5 almost always on 2-5
6 on 3-1 if you are healthy and have pairs and roll to about 20 to 30 gold.
or go on 3-2
7 on 4-1
and then 8 on 4-5 or 5-1 and roll till you are strong.
In Diamond this leveling pattern will change as you play with people who understand tempo so many hit 8 at 4-2
So the best way to play is to first understand Kai sa comps.
Urgot with RH LW and GA
Yone RH LW QS
Lux only if early yordle arcanist and SHE NEEDS BB IE JG
or Jhin with any ad items and bodyguard trait = usually sion blitz braum frontline
AT ALL POINTS IN THE GAME YOU NEED TO HAVE A CARRY AND TANKS
EARLY GAME TANKS: TRUNDLE, DARIUS, CAMILLE, KASS, BLITZ
AD ITEM CARRIERS: TWITCH, EZREAL, TALON, QUINN, TRISTANA, SAMIRA
AP ITEM CARRIERS: TF ZIGGS (FALL OFF AROUND STAGE 3) OR MALZ AND MISS FORTUNE
You MUST BUILD MORELLOS MUST MUST MUST MUST MUST HAVE
You will play best board until you roll at 8, but you should slowly transition into your final team comp in stages 3 and 4. Never hold early units early, and DO NOT WAIST RANDOM GOLD ON ROLLING, Almost all gold should be spent leveling until 8, ONLY REASON YOU ROLL BEFORE THEN IS YOU ARE HOLDING PAIRS AND ARE GETTING STOMPED
panda13 is right Kai sa with GA and Morello is probably the best carry obviously two star, but even 1 star kai sa with those items can pop off and win fights.
Please though do not try to force 3 socialite Kai'sa unless you actually know how to play it, that final board is so hard based on skill expression and hte socialite hex.
Better to just play Kai'sa with a good frontline, but if I have GA and Morello I buy Kai'sa EVERY EVERY EVERY TIME