actually u (generally) use fester (and other cds like painflare/bane) to get into DWT - while in trance its better to use trisaster+garudas con to extend dot-duration, than spam ruin lll (and maybe a shadowflare/miasra combo). Using Fester in DWT should just be the case if u fuck up your rotation cause of mechanics or stuff like that, other way ruin lll is your DWT go to spam-skill. After Deathflare/trance mode your dots should still be up cause of rudas con so you can use fester/painflare again.