"Powerful, tough, not so bright. The Oni Troll is a brutal figher. For maximum impact, use a Commander to control them."
The Oni Troll is unlocked by beating the "Prison Riot" mission in the single-player campaign.
Oni Trolls are slow moving tanks that soak up damage, providing cover for Mongols, Samurai, Archers, and other troops. Since they are so slow, they tend to bunch up and are best paired with Rush Commanders who can make them move significantly faster. While their clubs are often overkill for buildings and units, they make short work of walls in groups, providing a path for your archers and magic troops to move through to deal damage.
The Kami Tree provides two upgrades for the Oni Troll, upgrading damage and move speed. While the increase in damage is not particularly useful, the upgraded move speed is huge for these lumbering oafs. When fully upgraded, they are a force to be reckoned with. Once a high level... Can really pack a punch!
"Imbues the Oni with Frost Immunity, and the chance of freezing his target every hit!"
Oni have two options for a weapon, the first is in the above "Stats" table and is the "Iron Club". The second is the Frost Club which does less Damage Per Second than the Iron Club but has a chance for causing extra frost damage to its target, freezing them. It also provides immunity to being frozen (but not chilled) at all levels.
Unlike some other weapons, the Frost Club is almost universally better than the Iron Club. Immunity to the freeze effect from Freeze Towers is huge on high level bases, which can keep units frozen forever. The health and damage loss from the weapon is negligible since literally nothing stops Frost Club Oni from wreaking havoc on defenses except for their painfully slow walking speed.