Higher level Casinos 26-35
Higher level casinos are only available in Governor's War (GW) and beating them will often require that you squeeze every last drop of damage you can get. The tips bellow will focus on this exact thing.
Troop Type
By this point you should know the counters (Knife beats Gun, Gun beats Bike, Bike beats Knife). Recon the Casino you want to hit, the Troop Type you want to send is the one that counters ("beats") the Troop Type in the Casino. Troop types in casinos go in cycles (Lvl 26 has Knifes in it, 27 has Guns in it, 28 has Bikes in it, 29 has Knifes in it again and so on).
Troop Count
One thing that can improve your odds of beating a Casino is to maximize is the number of troops you send. This can be achieved by the following means:
Crew Size Boost
50% Crew Size Boost can be bought from the broker for 5k Gold. 
Larger Crew Size Boosts can be acquired from specific events.Active Skill "Mobilization"
An active skill unlocked through the "Battle" tree of the GW Specialties that grants you a 10% Crew Size Boost for 1 hour.GW Specialty "Casino Limit I" and "Casino Limit II"
Enforcers
Each attack can have 5 enforcers and each enforcer leads a squad of the same Troop Type as the Enforcer. Unfortunately for our 1-type attacks only 4 squads of the same Troop Type can exist in one attack. This means that if you use 5 enforcers of the same Troop Type only 4 will actively take part in the fight and grant you their bonuses. To counter that you should send 4 enforcers of the primary Troop Type and one enforcer from another Troop Type. For the last enforcer to take effect you should also send some (usually 5) troops of that type. Primary enforcers should have bonuses for the specific Troop Type while the secondary should give "Crew" stats.
Investments
You can improve your chances at a succeeding by investing in the "Casino" investment tree. There you can invest in improving the ATK/DEF/HP of a specific Troop Type, improving the ATK/DEF/HP of all Troop Types and reducing the ATK/DEF/HP of Casino troops (all the above applies only to casinos).
Additionally, assuming the "Crew" investment tree is already 100%, investments in the "Formation" tree can provide ATK/DEF/HP boost when enraged. Investments in the "Governor's War" tree can provide improvements on the ATK/DEF/HP of specific Troop Type or (quite a bit later) on all Troop Types.
Finally investments in the "Equipment" and "Super Weapon" trees can also provide a boost in these stats.
UB-related buffs
One can get buffs for holding and executing enemy Under Bosses (UBs). The execution buff is 38% on ATK/DEF/HP (the UB level affects the duration not the percentage) while the "holding" buff is 30% on ATK for a lvl 60 UB. It is generally discouraged to hold a UB while in GW since it allows players from anywhere in the map to raid you.
Super Weapons
Weapon placement is also important. If you are sending a mix of Grand and lower quality Enforcers give any weapons that that provide benefits based on the Enforcer's ATK to your Grand Enforcers. Also, you should give your secondary type Enforcer (see Enforcers section above) a gun with an effect that applies "at the start of the battle" as this enforcer won't stay in the fight for long.
Crew ATK boost
Crew ATK boost items can be acquired through events and range from 20% to 50% and 70% with various durations.
Attack strategy
After you have decided your Troop Type, Enforcer layout and Super Weapons layout and (optionally) gotten an execution buff, locate the Casino you want to attack and give yourself some distance from it (2 min marching time is a good distance).
Make sure to wear appropriate gear (boosting the stats of your primary troop type) and switch any jewels to ones boosting the stats of your primary troop type.
Set all specialties to boost the ATK/DEF/HP of the troop type you have selected and the rest of the points in the "Crew" stats specialties. Set all your GW Specialties down the battle path, for now it doesn't matter what troop type you choose as long as you reach as far down the tree as possible (at least until the "Enraged ATK" specialty and ideally until the "Equipment Augmentation" specialty).
Activate the following active skills (gun icon bottom right, where quick rob is):
- Mobilization (10% Crew Size boost for 1 hour)
- Enraged ATK (Gain enraged status and 20% ATK for 30 minutes)
- (Optional) Equipment Augmentation (Equipment attributes are enhanced by 5% for 30 minutes)
Activate your Crew Size Boost.
Reset your Governor's War Specialties and set them all down the "Enhancements" tree (ideally reaching "Casino Limit II" but at least "Casino Limit I").
Send your march towards the casino (5 troops of the secondary type, fill the rest with your primary type). If you've done everything right up to here with a 50% Crew Size Boost you should have 448k crew in your march.
While your troops are marching reset GW Specialties again this time filling the first two Specialties of the Enhancement tree ("Casino Expedition I" and "Casino Crew ATK") and the rest on the "Battle" tree but ensuring to spend points only in the Specialties boosting the stats of your primary troop type and the "Offensive Crew ATK/DEF/HP" Specialties. Also, this is the time to use any Crew ATK Boost items you have.
Example Loadout (Enforcers, Guns & Troops)
Assume you are going for a level 34 Casino which has bruisers inside so you are going to attack it with bikes.
Your enforcers could be:
Medusa
Buff: Crew ATK/DEF (or CREW ATK/HP)
Gun: RK 95 TP (Because it has a fixed stat boost at the start of the battle)
Tony
Buff: Bike ATK/DEF
Gun: M82
Cee-Jay
Buff: Bike ATK/HP
Gun: M4A1
Captain
Buff: Bike DEF/HP
Gun: MP7A1
Bubba
Buff: Bike HP & Crew ATK
Gun: FAL
With the above Enforcer layout (assuming all are level 60) and following the steps in the Attack Strategy section above you should be able to send 448,000 troops in your attack (5 Knives, 447995 T4 Bikes).