Wave 1 (Gekko) - Just wait for him to charge and then parry. Doesn't even need to be perfect. Leads you into the Zandatsu QTE almost instantly.
Wave 2 (2 x Mastiff) - Same kind of deal. Parry them perfectly and you will be put into the QTE. Try to do it so that when you parry, your follow up move takes out BOTH of the mastiffs. This will save you both time and effort, as well as reduce the risk of being clocked out of the game by the remaining mastiff. For me, the easiest moves to parry were the ones where they jump onto the wall, and then onto you - OR the ones where they running kick you.
Wave 3 (1 x Fenrir, 2 x Hammers) - I approached this randomly. My biggest thing was trying to get the Fenrir to take out the hammers. He has a move where he jumps back and throws four darts at you or something. It is possible for the Fenrir to take out BOTH of these guys if you position it properly.
Once you've taken them both out, L'Etrange will actually stall the Fenrir quite a lot. The best way I did it was to build distance between me and the Fenrir until it tried to use it's beam attack. Then I would run in before it could - it would cancel the use of the move and by the time it can react you can slide kick + follow up and then hammer him with L'Etrange. L'Etrange, when fully powered up, will also ensure no Dwarf Gekko will attach on to you. Just don't get greedy with your attacks - Fenrir will jump out of them at some point and do a wall bounce attack (getting hit by this is a death). Basically take it slow and be careful.
If the beam is giving you trouble (it was for quite a while) then take the time to slice the gun off when you can. You will build your blade mode up super fast using L'Etrange against him anyway.
The reason I recommend taking out the hammer guys first is that they will sneak up on you while you're pummeling Fenrir. Of course, you could take out Fenrir first (this was my initial approach) but given how quickly and easily Fenrir kills the hammer dudes I went for my own approach. It was also easier if all I had to focus on was Fenrir.
Wave 4 (4 x Dwarf Gekko) - LOL
Wave 5 (1 x Big Daddy Thing) - This one is easy too, just Ninja Run whenever you can. Run in and hammer him with L'Etrange. He will attack you - parrying is easy in this situation. Use Blade Mode wherever possible. The GRAR or whatever the fuck it's called was actually the easiest part of this mission for me. Not sure why, but yeah, maybe because I had just taken down a Fenrir.