When you go into a fight in an arena, you should pick a melee target that cannot cleanse your dots (ie, PT, Jug, Mara) Then do the following on that target:
Creeping terror > Discharge > Recklessness > Death field (Make sure you have at least 2 in the range, preferably 3 and make sure sins don't have their shroud up)
This will spread your dots on everyone your death field hit as long as your original target was also hit. Then pick your target, preferably a sorc/pt/merc (Someone squishy). Make sure you have your dots on that target, especially discharge as it will proc your instant demolish. Keep dots rolling on that target and use the following priority rotation:
1) Assassinate (Only on proc or below 30%)
2) Demolish (Only on proc)
3) Leeching Strike (Off CD)
4) Thrash
Do not use shock unless your target is out of melee range or you need to get rid of a recklessness charge. It would almost be better to remove it from your toolbar as it takes up too much force in the hatred spec. Always make sure to refresh dots before death field comes up and spread those dots to nearby targets with death field. By doing this, you should see your damage increase significantly. Mind you, this is just a general rotation guide and everything is situational, especially in arenas.