Optimizing the monsters.

[Lupine]

It's okay to let players have a session or two of domination. As a player, I certainly wouldn't want EVERY session to be that easy, but once in a while, it's cool.

Mostly this.

I will say, you can optimize without having to change whole lot. For example, Lizards start attacking with hit and run ambush attacks. Lay some traps down for the party, put down some fog, and hurl javelins at them. The goal isn’t do much to kill them, but to wear them down, and whittle their resources.
Keep these attacks up. They hurl javelins, and then retreat, hiding. Once the players lower their guard, do it again. When the players get hurt, they heal up, but this slowly drains their magic an resources. Eventually, they must turn back, or risk resting overnight.

A particularly nasty trick would be to, after the party has been ambushed 5-6 times, make a deadly trap:


They begin to hate these lizardfolk. The most perceptive member spots a lizardfolk crouching in the underbrush. They charge it mindlessly, trying to prevent another ambush. A fog cloud envelopes them. As such, they never get a chance to see the pit fall. 30 feet down and several poisoned spikes later, the lizardfolk arrive, and nail the players wigh javelins. They can’t fight melee, and the fog cloud means they are effectively blinded. The lizardfolk, however, peek in just long enough to throw a javelin, then retreat to total cover, away from the wall.
Players have to fight virtually blind, on the low ground, with little ability to get up.
Does one character use spider climb? Lizardfolk swarm him, now that he has no allies. When he goes down, they continue as normal.
Use a number of Lizardfolk such that each player should be hit twice per round, on average (dmg mob rules) before any die. Have them target one player (the healer), though if a player tried to climb, or otherwise get out, then target that player.

No monster buff is needed. Just more of them, and better tactics.