hmm i guess i don't agree w/ most of the above suggestions. i wouldn't replace chansey with those fairies since they're just as passive as chansey is. if you're running a bulky offensive team, i'd use more aggressive mons than unaware clef etc. also don't agree about altaria. i actually really like the bulk on the altaria since as u said keld is a huge huge threat. i don't see what the extra speed is actually good for since outpacing jolly azumarill is plenty good for the purposes of ur team. u can adjust the power if u want but i think it's fine.
jumping straight into a chansey replacement, i think a lati would be good. it's cookie cutter, but you'll have an answer to zard y still while getting either a useful attacker (3 atks) or a utility mon via hazard control (2 atks). pursuits are free switchins for altaria though it's still a problem.
speaking of the hazards game, the most glaring flaw in the team is a lack of stealth rock support. stealth rock is
MANDATORY on every serious team. the damage it racks up and the pressure it can put on opponents (ie putting certain mons on a timer) is crucial. i actually don't like gastrodon here. like chansey, it's extremely passive, and it just isn't *doing* anything. i think something like hippowdon would be an acceptable replacement as a defensive backbone (rly helps against lopunny especially) while giving you this crucial move in your arsenal. eq/sr/slackoff/stone edge is an acceptable choice that also gives you an answer to non-wow (and maybe bulk up? idr who wins 1v1) talon, which is better than nothing i guess. if you want a more airtight answer for talon, heatran, tyranitar, etc. are acceptable as well.
anyway, once you get in the hazards game with team should work a lot better.
also, gonna echo the stuff about replacing energy ball on gengar (you're actually super sd gliscor weak so i'd toss icy wind back on it to lure it), aerial ace on sciz (use superpower), etc.
Well, now that you mention it, bulkier Ground Pokemon Therian Landorus (or Hippowdon as you said) should be used over Gastrodon; both get Stealth Rock, while Gastrodon doesn't:
Therian Landorus:
-Stealth Rock
-Earthquake
-U-turn
-Stone Edge/Knock Off/Rock Slide
Nature: Impish
EVs: 252 HP/240 Def/8 SDef/8 Spe
Item: Leftovers/Rocky Helmet
or
-Stealth Rock
-Earthquake
-Explosion/U-turn
-Stone Edge/Knock Off/Smack Down
Nature: Adamant/Jolly
EVs: 72 HP/252 Atk/184 Spe
Item: Normal Gem/Soft Sand/Earth Plate
Mega Altaria is a very good Heatran lure. With a Timid nature and max Speed, it can outspeed all Heatran variants and hit it hard with Earthquake, even with a -Attack nature:
0- Atk Mega Altaria Earthquake vs. 0 HP / 0 Def Heatran: 268-316 (82.9 - 97.8%) -- guaranteed 2HKO
0- Atk Mega Altaria Earthquake vs. 248 HP / 0 Def Heatran: 268-316 (69.6 - 82%) -- guaranteed 2HKO
Have a pick between Latias or Latios. Latias can provide Healing Wish support for any crippled attackers, while Latios has more power behind its attacks:
Latias:
-Draco Meteor
-Psyshock/Hidden Power (Fire)
-Defog
-Healing Wish
Nature: Timid
EVs: 72 HP/184 SAtk/252 Spe
Item: Life Orb
Latios:
-Draco Meteor
-Psyshock
-Surf/Hidden Power (Fire)/Earthquake
-Defog
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb