>This is on vanilla settings (and not challenge runs), yes?
Yeah, just plain Hard mode. There's two easier difficulties, but I haven't tried them and anyone interested in the DLC should start with Hard, since the DLC content is substantially harder than Hard mode.
>Fire causes them to explode and water puts them out.
Hitting them with fire damage causes them to grow and become a bit stronger up to two times. If they have already grown twice they will instead self-destruct. The self-destruct can damage other enemies and also deals fire damage. The more powerful Bomb variants, Grenade and Neutron Bomb, do not self-destruct, and on the DLC difficulties Bombs no longer self-destruct either.
Water can put out environmental fires, but does not have any special effect on Bombs as far as I know, they just take bonus weakness damage from water. Most enemies have a weakness to at least one of the 8 magic elements and one of the 3 physical attack types.
>enemy interactions
Well the Bomb thing is pretty unique, I can't think of any other normal enemies reacting to a particular element like that. But most enemies have a gimmick of one kind or another. Sword-wielding skeletons can block and counter melee attacks, while spear skeletons can block and counter magic attacks, and in both cases their counterattack is a copy ability, so you can bait the counter in order to steal the copy ability. Gigas (giant humanoids) have a big scar on their back, and dealing enough damage to it causes them to collapse into a stunned state. Thunder magic is handy against them because it strikes from above, hitting the scar no matter which way they're facing. Griffons start out wearing armor that gives them a super armor effect, and when it breaks they become much more aggressive. Wolves come in packs with a larger alpha that can command the others.
More generally, there's a big emphasis on knocking enemies around and utilizing the environment. Aero magic is very very strong because it carries enemies a long distance, making it perfect for getting wallsplats, and if the enemy is already close to a wall, Aero will ram them into the wall repeatedly, dealing several wallsplats in a row which is pretty much guaranteed to put them in a break state. Stages have environmental features that you can use as well. Pools of water can be electrified, pillars can be knocked down onto enemies (or onto you). One stage has clouds of explosive gas that are dispersed by wind or detonated by fire.
This seems an appropriate time to mention the chain soul burst mechanic. When you execute a broken enemy, their body explodes in a 'soul burst' which gives you a chunk of MP. Where it gets cool is that the explosion damages and knocks back nearby enemies, possibly causing wallsplats for further damage, and any enemies who are broken or killed as a result of a soul burst will also soul burst. Small enemies are often killed by chain reactions in this way.
Bosses all have plenty of unique stuff going on. Lich summons skeletons to attack you and then uses their deaths to power a Flare spell, Kraken has a grab that steals your buffs, the Dragon Zombie makes puddles of poison that are destroyed by holy damage, etc.