Story related, unmissable.
Obtained at the end of Chapter 12: Where Crawls The Lizard? after defeating the final boss.
The first time you see the Lizard in this level will be whilst protecting Gwen, who is fixing the Hunter robot. Defeat all of the Infected in this area, and he will jump down and attack Gwen. Web strike him with and knock him away. Repeat this three times, defeating all the Infected each time, and he will run off.
Fighting the Lizard in the next stage is deceptive. Hit him when you can, and you will eventually web him to the wall. Now ignore him and run to the Hunter robot and hammer the button to pull the wreckage apart. If the Lizard escapes, and he likely will, press to dodge him and web him back to the wall again. Once you’ve pulled the wreckage apart and found the serum, you’ll move on to the final area.
Attack the Lizard in the centre, and then web shoot him to slow him down as he runs around the outside. Web strike him when he is stuck, and then move in for some more punching. Dodge with when you need to, and then press repeatedly when prompted to try and administer the serum.
Lizard will knock you off, and you will need to web rush with to recover. Repeat the process, and this time you will get the serum into the Lizard’s neck, and the battle will be over. Well done!