A generalized tactic per colour note:
W: Weenie tactics. +1/+1 and -1/-1 and redirection of damage to win combat against odds.
U: Hand disruption, slow down of opponent creatures and Homorid/merfolk midrange creatures. Excellent utility colour with strong blue vs blue bombs as well as Deep spawn and Leviathan.
B: Hand disruption and destroy effects with self sacrifice.
R: agressive and strong midrange while unable to block. Pair with war drums to even out.
G: Defensive walls and elven fortress and slow thallids. Finish with big craw worms or lots of saprolings.

Vs White: Martyr's Cry, Nameless race, Raiding Party
Vs Blue: Riptide, Thelon's Curse lots of large creatures if opponent control an island
Vs Black: Karma, Thelon's Chant
Vs Red: Volcanic eruption
Vs Green: Tourach's Chant, Hidden Path

Vs any colour: Psychic Allergy
Vs Goblins: Tivadar's Crusade

Repeatable destruction: Merfolk Assassin+War Barge

Another tip is Magical Hack to allow for giant shark or sea serpent attacks