I just fought this team twice in today’s GW with my main yellow team and had not much trouble. In my case, Mab was in the bottom spot. Since Kraken is slow to power itself, knocking out Forest troll is a priority. I used
Yellow/Green banner, all cards fully traited
In the first match, one of the Krakens managed to devour FG, so I devoured it with Maw in a fit of revenge bloodlust. Still, I lost a card, well, it happens against Krakens. In the second match, I did not devour anything as I had to cast Green Seer on yellow and then FG eliminated all brown. But there were enough skulls and some cascades to kill everything in a couple of turns. Finished with 4 cards intact.
Initial goal is to power up Seer with purple trying to keep some brown intact, cast Manticore if absolutely safe to stun Mab or to drain mana from Troll or Kraken. Then, it should be more or less smooth sailing but have to pay attention. Of course, if possible, power up Maw but it might take awhile unless really lucky with the starting board.
In general, Manticore is one of the most essential cards for GW IMHO. Especially when facing Mab, makes life easier.
On a green day, Manticore - Giant Spider - Forest Guardian - Kraken works fine as well, usually no major issues even with bad starting boards.
Red day is still quite tricky for me, it is probably the most difficult and less reliable. I have 3 different teams but they all stumble occasionally. Blue, brown and purple should have no problems against Forest Troll - Kraken combo.