Perhaps consider giving the character a Foestone? (Level 12 Wondrous Item - minor action, at will, to know a target's vulnerabilities and lowest defence)
I had to go look that up. That is an awesome item.
... Weird. The monk and mage have the best chances against the big bads (12+ and 11+; 10+ and 9+) and roughly equal chances against the rest.For an At Will Power for the Players in game at present-
Fighter #1 +17 vs AC
Fighter #2 +17 vs AC
Cleric +16 vs AC
Assassin +15 vs Fort
Mage +15 vs Ref
Monk +14 vs Ref
...
Last fight was in the Pyramid of Shadows-
Charnel Lord 15 Elite Soldier AC/F/R/W 30/32/26/29
Over a dozen Charnel Rats 12 Minions 26/23/26/25
And throw in a Level 12 Elite Obstacle (Trap)
...
Fight before that
Gharash Vren, Dragonborn Soldier Level 15 Soldier 32/27/24/27
3x Human Archers Level 12 Artillery 24/23/24/22
3x Human Outlaws Level 13 Soldiers 29/26/25/24
And a Level 13 Elite Blaster Trap
Vs the elite soldier my monk build would needHowever I've been throwing +6 level creatures at the party for the best part of 6 months, and we've had no problem so far.
For an At Will Power for the Players in game at present-
Fighter #1 +17 vs AC
Fighter #2 +17 vs AC
Cleric +16 vs AC
Assassin +15 vs Fort
Mage +15 vs Ref
Monk +14 vs Ref
Rogue +18 vs AC
Last fight was in the Pyramid of Shadows-
Charnel Lord 15 Elite Soldier AC/F/R/W 30/32/26/29
Over a dozen Charnel Rats 12 Minions 26/23/26/25
Vs Gharash VrenAnd throw in a Level 12 Elite Obstacle (Trap)
Anyway 6075 XP Level 14 Encounter
They wiped the floor with it- five rounds, lots of bloodied, a daily or two got used but they all kicked ass- save the Monk, who failed to hit, sure he was in a pit with the Otyugh but so were three of the others.
Fight before that
Gharash Vren, Dragonborn Soldier Level 15 Soldier 32/27/24/27
3x Human Archers Level 12 Artillery 24/23/24/22
3x Human Outlaws Level 13 Soldiers 29/26/25/24
And a Level 13 Elite Blaster Trap
Total 7300 XP Level 15 Encounter
Guess what, five rounds to rip apart, although Gharash surrendered when he was bloodied and the last bad guy standing.
I've thrown Level 17 encounters at the group (3 of them to be exact- I keep copious notes down to every last XP point I've dished out, every creature they've met and interacted with, or simply fought and/or vanquished- an Excel spreadsheet with lots and lots and lots of info- I'm a Higher Education (Degree/Masters) level lecturer by trade with +10 in anally retentive).
IMHO the problem of the published monk is: single target at wills all attack vs fort, which is quite unfortunate as most big single target enemies are brutes or soldiers with high fort.
A burst-1 enemies only Dex vs reflex power, at that, and one that provides a sweet, sweet shift 2.One is even a burst-1 power.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.