To beat the doomkitten easily for rouges you have to buy the best water weapon you can
find.
To see whats the best for your level and class see here
http://forums2.battleon.com/f/tm.asp?m=13420620

For level 30+ rouges its Rip Tide which you get from Red Betty’s Spell.

Ok what to do is get Artix and another guest on your team then get your baby dragon. Make
sure he's water. Equip your best items + rip tide or other water weapon. Go verse the
little bugger. use the following combo:
Final
Rapid
Stealth
Surprise
Rapid
Smoke
Heal if needed (can be doen before here)
Blind/Normal
Rapid
Normal
Rapid 
Just keep following until hes dead. remember to use smoke and potions whenever you need
health