Tips for Acorn character challenge-
~For the role for damage challenge, you have to hit multiple targets with one role for damage. I found ops to be very useful for this, as there are sometimes very large groups of weak zombies and role for damage regenerates relatively fast.
~For the sap trap vanquishes, I played a lot of PvP and PvE and used upgrades that help sap trap recharge faster. There is an upgrade that makes sap trap recharge faster if it doesn't deal any damage, and I found this useful for missed shots. I didn't use oak's sap trap for this challenge, as his trap recharges waaayy too slow.
~And for the land hits and earn experience, I started a battle in PvE and just used acorn and not oak, as his rate of fire is much faster.
I'll admit, I wish they did something with a challenge that would border along working together with other acorns, such as healing oak blank times. For some reason I cannot remember what the last challenge was... I'll get online soon and update this later.