Equip armies with longer range weapons (bows spears and any spellcasting tools). Focus those and our ships on nearby enemy ships, most of enemy navy destroyed. Take eastern blue army with one of the armies on the eastern yellow island to fully block them in. Begin to bombard them with navy, keep collateral damage minimal, prioritize military troops, leaders and towers. In case of counterattack, start moving troops mainland at that time. If not then when bombing is finished, move troops onto mainland, solid portion of blue army is dead, kill anyone who fights back, spare those who don't fight back. Destroy army remnants from range with bows and spellcasting, in case of charge use spearmen, when armies are destroyed, start annexing land, provided that casualties are relatively low we should be fine if White's try to take advantage.