With no Fortress, it's not your L that takes damage, it's the entire party.
You'd be wasting your time trying to make an L into a tank. They're durable in the sense of not needing to be babysat like a Bloodsurging Nightseeker needs to be, but they're not made to tank. If you try to turn L into a tank, like maybe by subbing F, well, they can sort of tank, but you'd be better off with just having an F in the first place in that case, because when it really matters the L won't be able to tank as well as an F and if it's trying to tank it won't be able to do the kind of damage it's designed to do with link setups.
There aren't many enemies in the game that can kill you with one hit. If you have either a Medic's strong healing or a Dancer with skills allocated for healing and/or an Arcanist subbing Medic, you will probably be okay for most of the game with no tank but your healer unit(s) will be working harder. Some of the tougher bosses may give you trouble if you are used to having a tank in your party's lineup, but people have cleared the game with no tank, it just takes a different approach from how I played the game. I typically take my F with me when hunting postgame bosses in EOIV since I'm used to the flow of battle when she's present so I'm not sure what the good strategies are for fighting them without a tank, but I know people have done it.
You really can't go wrong with Nightseeker.