Gym strategies

It depends on the layout of the gym.

If there is a Blissey, I take an "Avoid Blissey" strategy. I take down everything below Blissey, then throw in as much firepower as I can in the next run, get through as many mons as possible, and hopefully only have to face Blissey once, or twice at most.

If there are a bunch of Rhydons/Vapes or Gyrados/Vapes in a row, that's a good layout to make a long run. Set up your big grasses (Rhydon) or electrics (Gyrados) and plow through as many as you can.

My other strategy is to collect 200+ suicide monsters (700CP+, stuff you don't want like Furrets, Raticates, Ariados, Eeveeloutions with crap movesets...), and just blast away at everything but Blissey. You'll only get about 2-3 defenders per run, but with 200+ of them, you might just take down a L10 gym with no potions or revives. Don't use your suicide squad on Blissey though - you need real attackers for that...

If it is a lvl10 gym...hit the first mon once and get out.
Then you are faced with only 9 defenders in the gym and the odds of you getting through from bottom to top in a single run is a lot better.

I use 3 dragonite and 3 gyarados to run thru almost any gym. I only dodge charge attacks to make it faster. If there is a bunch of blissey then I add some fighting type. I do like the avoid blissey tactic mentioned so I might start using that.

Depends a lot on your level. Here is something I found very interesting on reddit, among the comments of https://www.reddit.com/r/TheSilphRoad/comments/634xx2/a_more_comprehensive_take_on_an_attacker_tier_list/
"What glass cannons do better...
When you solo a lvl 10 gym, always start with the mon that might be a little slower, but survive longer.
After the first pass, adjust to ideal specialists given the movesets which you now know.
After a couple more passes, you get near even in numbers, at that point strategy shifts from surviving to get the 1K bonus to raw speed, as there is little risk of missing the 1K bonus.
Once you get ahead of the numbers count, DPS reigns surpreme. Load up the glass cannons and quickly finish off the gym.
While most people investing in their attack lineup will start with the lvl 10 lineup (its the most versatile), eventually you get to where you can buff a high DPS assault force.
Now a reply:
Generally I do the exact opposite successfully.
I typically use a glass cannon, usually Alakazam, as the lead, followed by Dragonite, then the remaining four on the team focused on bulk and type matchup. The reason for doing so is that the glass cannon plows through the "trash" bottom 3 or so quickly making taking down the rest of the gym trivial.
Of course, it all depends on the area you play as to gym makeup as well as trainer level.

See if suits your pokemon bag and gyms composition in your area.