Metamagic broken?

Is it just me or does it seem like metamagic feats are a little broken right now?

A level 6 Cleric almost 1-shotting an Osyluth (bone devil; listed at level 10)?
Or is the bestiary just incredibly weak right now?  Maybe I'm over thinking it. 
You are going to have to explain that. The strongest 3rd level spell so far is the Druid's Call Lightning which is 4d10 Dex half, 6d10 if outside in a storm. The Cleric lacks offensive spells at level 6 unless an Arcanist or Storm Bringer at which point the best they have is Lightning Bolt for 6d6 Dex half. If using both Heighten Spell and Maximize the Bone Devil saves against DC 15 with their +3 and Disadvantage, probably failing, taking 36 damage ~ a third of their 95 HP. The trade off is the Cleric used one of their two 3rd level spell slots for the day and both metamagic feats, taking up 3 of their daily uses. 

Yes the beastiary is actually a little underpowered but I think you may be looking at something incorrectly as well.