Since your Suicune is Bold Natured, you can run the CroCune Set...
Suicune - (Holding Leftovers)
252 HP / 252 Defence / 4 Special Attack as the EV's
And the Moveset is; Scald / Rest / Sleep Talk / Calm Mind.
(Set is stolen from Smogon)
It takes just about every hit, and if you can set up with Calm Mind you'll be doing a lot of damage. It isn't very offense based, but it's good.
But if you want something more offense based...
You should probably (if possible) run a Modest Nature, with 252 HP / 252 Special Attack / 4 Defence / as the EV spread.
The moveset should look like this; Scald / Ice Beam / Extrasensory / Calm Mind /.
Shadow Ball, and Signal Beam are other options. and Surf (in place of Scald).
The item should probably be Leftovers, so Suicune can recover some health each turn.
Hope I helped!~