Should I replace Sk with that DD or should I replace it with Rockno/Trix/Grim?
If you don't have Sasq than Rockno will be great addition to your team instead SK. Just make sure to have Celestin pet, to remove buffs from opponent, especially enemy SK. On other hand if you are seeing many SK, than pick GF, as he will be able to handle SK quickly. It would be best if you could use both of them, but i am assuming you have only 5 Hero Bases (which means you need to focus to get 6th as soon as possible).
DD is good tank, but since you have Anubis you don't need another one (except Rockno, which is good also in terms of dealing area damage to enemies.
As for now if i were on your place i would test Rockno and GF and see which one suits you better. Leave DD alone for now, focus on your main team.