Here is the full list of all 15 Plexarium achievements.
Crawled the Plexarium in practice mode
Crawled the Plexarium in time trial mode
Crawled the Plexarium in endurance mode
Picked up a phaser
Crawled 5 levels
Crawled 10 levels
Crawled 25 levels
Crawled 50 levels
Crawled 100 levels
Crawled 150 levels
Crawled 250 levels
Trapped a spider
Deployed Radar
Deployed Arbiter
Deployed Talon