Can anyone give me some tips to fighting crabmen in towns?
Early in the game a town came under attack from crabmen and I’m trying to clear them out.
However, it’s really fucking difficult! It’s a slum/shack town and it’s very cramped. The problem being my men can barely get any line of sight of the crabmen, but since they use grenades they’ll just pelt me with grenades from all sides and I can’t seem to return fire at all.
I keep getting destroyed and I can’t really figure out how to handle it.
The best answer to grenades or explosives in general is to spread out so they hit less people per shot.
Try getting snipers on high ground so you are more likely to find a line of sight (then you blow their grenade arms clean off with a well placed bullet). Get explosives of your own: equip your heavy with a grenade and missile launchers, get grenades to your troops.
If the crabmen lack the machine gun arm, ignore cover altogether: it won’t do you any good vs grenades and claws anyway.
Well they usually don’t shoot with grenade untill they see your soldier. So try to hide at least 2 squares behind building corner. They will need to run closer to spot you. Then use your soldiers which stay more behind to shoot them. And those standing closer to the enemy leave with overwatch. And also use grenades with all your soldiers (not just Assaults). They are quite cheap and effective.
Two things I learned while playing.
The crabmen in my first mission were very high level compared to what they usually should be, there was a lot of them and they ALL had grenades and in the best possible site for them.
Since then it’s been a lot more balanced, but damn if I run into that combo again - ouch, that’s just mean.
Enemies are really random right now. But your soldiers after reaching level 5 are killing machines. First missions can be quite difficult if randomness thows something more dangerous at you. But well this is un-ballanced pre-alpha build of the game. 
I love those missions, sad thing is the civilians that always run towards the Crabs. I would love some kind of mechanic that made me be able to talk to them and get them in safety.
My setup that crush anything is 4 Core Inf, 3 snipers, and either a heavy or a Mec. When my core inf or sniper reach new class level I make them heavys and take perfect fly.