Ok,
Overview:
You need to go into specifics on a lot of things. Why is the typing bad defensively?
What makes it a great staller? Why is Dream World ability a let down? etc. The answers may seem obvious but are necessary for people new to competitive battling.
Mention something about its decent Sp. Atk and how it can use its STABs effectively with it. Mention how it is crippled by all hazard specifically T-Spikes and SR. Mention how it needs a lot of support (T-spikes users, Rapid Spinners, Hail, Spinblockers for T-spikes users etc.)
Set Comments: Math is scary but convinicing. 12% + 6%= 18% x 2= 36%> 25% (look at how pretty that looks) dont use that though, comment on why this set is used (being able to recover more health than lost from Substitiute). Go into the use of Sub and Protet, Sub protects from status and Protect allows you to get an extra 18% of health but be wary of set-uppers. Talk about the 32-turns Walrein gets of PP, its kind of a must, iirc it can kill 2 or so full health Pokemon off with Toxic and Hail but check that. Talk about the low PP that Blizzard has. I wouldnt say Roar is useless outside of hazards considering how an opponent an easily predict a Protect and set-up so Roar allows you to defend against that.
Additional Comments: This is really barren, odd considering how much support Walrein needs. Hail support is obvious, T-spikes are basically required (which you have but needs to be emphasized). Rapid Spinner/ T-spikes absorber (Claydol has nice synergy and Cryogonal likes hail, Roselia can also absorb T-spikes too). Getting rid of poison/steel/ flying types is nice and Dugtrio can do that to poison and steel types while SR help control the latter. Talk about the purpose of the EVs (HP allows best Sub and Leftovers recovery, most resistances are physial so max defense with the rest dumped into Sp. Def.)
Other Options: Not many. Talk about Curse and how it is outclassed by Lapras. Brine is gimicky with Super Fang as it has 195 base power when the opponent is below 50% with STAB but Surf is more reliable.
Checks and counters: Once again, why do you mention Muk when there are so many other pokemon that are immune to Toxic and are neutral to his STABs too. All poison types in the tier are iirc, as well as majority of Steel types specifically Aggron who has a super effective, STAB, 150 base power attack to use on you. When you mention Clefable it is not all specially inclined pokemon, it is specially defensive and only those that arent hurt by Toxic like Drapion and Clefable. Walrein is super slow so a swift Toxic or Taunt means its bascially screwed. When you say phazers and mention Munchlax also mention Poliwrath for Circle Throw and Encore and mention how they fear Toxic. If hail is gone when facing a sun or rain team Walrein is severely crippled. Rhydon can break through Subs with Rock Blast and once again hazards suck.