Raith Marid is very good. I think Godtear is a really well-balanced game, but inevitably some models are going to be a bit ahead of the curve. It seems like the Moist Wyrm is a bit ahead of the curve. Some folks (especially British ones) seem to think that he’s the best Champion in the game. I wouldn't go quite that far, but I do agree that Raith is very good.
As a result of the consensus that he is very good, the Soggy Dragon gets played a lot. This means I see him a lot. Please let me share what I have learned.
DO
Kill the Splashlings. If you take one piece advice from this post, let it be this. Don’t worry about that boon. Kill those little bastards. Kill them with fire. This means that you need to have a couple of high accuracy attacks scattered throughout your warband, but that’s something you probably want anyway. The reason why you want to kill these puddle demons is that each of them represents a potential landing spot for Drippy Drake. The fewer splashlings, the more predictable Raith becomes. The big limitation on his awesome mobility is that he can only jump to spots that he has set up in advance. This gives you the opportunity to counterplay. If you commit to thinning the puddle ranks early, this creates problems for Damp Gecko which compound as the game goes on. This is because Raith has to remove a splashling himself whenever he Kersplooshes. If you kill two and he removes one himself, the splashlings have to recruit before Clammy Reptile can teleport again. This means that the splashlings won’t really be able to use their Tide ability (nice) and that Raith’s movement options are highly predictable and take an entire activation to set up (very nice).
Anticipate the Tsunami. The best tool in the Liquid Lizard’s arsenal is his ultimate. He essentially gets to teleport three hexes (no splashling needed) and then move all enemy models within two hexes of the landing spot up to two hexes in any order he wants. That last bit makes it especially devastating. Used correctly, this ultimate can not only win a turn, it can also put you out of position for the next turn. It’s probably the best ultimate in the game. It usually gets used on turn 2 or 3. So what can you do about it? First, spread out if the scenario allows you to. Second, backstop your champions with followers. Credit to James Doxey what for coining his term. It means, position your followers behind your champions in order to prevent them from being pushed away from where you want them. If you can predict Raith’s landing spot, backstop some followers three hexes away from that spot so that they will not be affected.
Chip away the Boons. If you have a low value attack (or other skill that can target Raith) on a follower unit, throw it at Raith to take out his boons.
DON’T
Obsess over killing him. The Slippery Serpent is actually fairly tanky. He’s got seven health and will usually have armor 4. He certainly can be taken out, but it is often not worth it, especially since he can escape from sticky situations very well. Instead, use your attacks to limit his options by killing his followers (see above).
Worry about him killing your champions. He can definitely kill you, make no mistake, but he probably should not get bogged down in a slugfest. His attacks are not actually that strong for killing champions unless they are buffed, but if he’s using his boons offesnively, you may well be able to retaliate and exploit his pedestrian defensive stats. I often put the Rainy Reptile on the table myself, and when I do, I’m generally trying to avoid getting stuck in. Fundamentally, if Raith is attacking your champions he's probably not squishing your banners. That's good.
Panic. He’s actually fairly manageable if you bear in mind what he can do. Just imagine that he can be within one or two hexes of any splashling. Try to see his potential position instead of his actual position and you’ll be fine.