Tree stump is the oldest coliseum level, and the major testing grounds for new heroes. Basically, it's a circle.
It feautures circularly-shaped mushrooms that grow over time... The mushrooms boost both speed and damage, but how much exactly does it boost depends on how much it grew. It grows slowly, and when it matures, it has a green circle around it. If not consumed a few minutes after maturity, it begins to shrink much faster than it was growing. There are also many narrow poles surrounding the middle and larger poles around the rim of the stump. Be very careful around these; an ability like Assassin Cut can be wasted by having Spider charge into a pole.
The enemies here do not include Siege enemies, like the other Coliseum maps. Instead, many swarmers join the fight. As the fight progresses, more powerful enemies join in too, such as Armor Ants, Elephant Snails or Bumblebees, etc.
Take it easy in the beginning, and save up for the better skills of your character. Right now, only a few swarmer-type enemies will spawn, so take the free money and make the most out of it. It's a good idea to get some cheap, low cooldown abilities or passive stat-boosting abilities to deal with the amassing swarmers, then buy some more powerful ones. Later on, you'll be assaulted by Elephant Snails, Bumblebees, and Queen Ants, which are extremely hard to deal with due to the limited amount of space to use to flee; bash them with your more powerful abilities such as Lightning Storm or Executioner. Also, in times of need, use the mushrooms growing around the area to your advantage, since they increase most of your stats greatly. However, it is wise to only use them when they are at least somewhat large, so that the boost effect is more powerful. The wood columns around the area can be used as a small amount of shelter from bullets, but won't impede melee strikes.