I think Aurora is a really good counter to Derisda. She locks her down with her non-ult spell for like 6 secs, which in arena is a long time. Aurora is also pretty cheap at like 50 bucks for a 6-1 and since she's Early Adopters she can use Cracked Hearts to level her up. One final tip on Aurora, since she's Umbrus she pairs really well with Tavros so in non-pvp she will lock up a mob and allow Tavros's heals to keep ticking.
