Tionese is fine as dps.. Anything above that is a bonus..
For Lost Island you should read the tactics, watch the videos and PLAY NORMAL MODE FIRST..
Once you know ALL the mechanics by heart, then you can try LI HM..
2 Tionese dps with a columi heals may not beat the soft enrage on lorrick. Also most group finder groups will kick an undergeared dps because it is easy to find a replacement.