You can gain XP in three ways; by killing enemies, completing objectives, and by collecting XP canisters in single player and Echelon mode.
Playing your favourite maps of Echelon mode can be the best way to gain XP (Deveraux's Hideout or Fuse Reactor being the easiest), but there is also certain sections of the campaign such as the second two checkpoints of Triton Outpost and the third checkpoint of Grigori Station that you can gain a massive amount of XP on.
You can also buy an 'XP Multiplier' team perk with Fuse Credits that will increase your XP gain in all areas by 10% at level 3. If you didn't get enough Fuse credits from the campaign, you can load up the second checkpoint of Triton Outpost - 'And I Gazed Into the Abyss', grab the Fuse Credits just over to the left, and repeat.
Gaining XP specifically with Dalton is all about using his Xenotech weapon; the Mag Shield. Hold
to activate the shield, it will block any and all bullets, grenades, rockets and other fire, you can either press
for a short range blast, or wait until you catch an explosive and blast it back at the enemy. The Mag shield is a short range weapon so you need to get in close and blast away for maximum XP.
Once you reach level 35 you will have the required skill points to fully max out his skill tree. You can assign skill points from the 'Agents' section of the campaign or just by pressing the
button during gameplay and going to the skills page.