Kaijuu 8-gou

Both World Trigger and Kaijuu nr.8 have a strong sci-fi-esque ambience, and if you're into that aesthetic, I recommend checking out either series! But that is not the only similarity! In both cases, the city is in constant danger from monsters appearing, and the protagonists are specialized in defending the city. Well, that's a strong similarity! But wait, it gets even stronger: while the standard weapons depend on the individual's abilities and creativity, there are some rare personalized weapons, that require the user's compatibility and that are made from other people's sacrifices. So, if any of those aspects captivated you in one series, you might enjoy it in the other title as well! read more 

Main character is stupid and endearing. A lot of blood and guts. Human society has adapted to a world in which monsters wreak havoc upon the world. Humans use the power of the monsters to fight back. Main character is part of the main organization that fights back against the monsters. 

Both MC's gain their power from the threat to humanity by combining with it in some form Both stories have humanity fighting some form of dangerous creatures. As their names suggest, in Parasyte Maxim, it's parasitic aliens while in Kaijuu 8, it's Kaijuus 

A similar plot outcome of the main character to those with whom he fought / wanted to become 

for those fans of yor... dont miss this anime... we get to see yor on steroids once more. in both very action packed anime... but this time we get a yor that kills kaiju. 

A world were mindless monsters constantly invade, with an organisation made to counter the threat. The stronger monsters have higher intelligence, and the stronger fighters of the organisation are forced to fight these monsters. 

Gantz and Kaijuu No.8 both delve into the theme of individuals harnessing advanced plugsuits that grant physical augmentation akin to a power-up mechanism, providing enhanced strength, speed, and durability in order to combat and overpower formidable foes, be it space aliens or giant monsters.